diff --git a/changelog.d/1020.change.md b/changelog.d/1020.change.md
new file mode 100644
index 000000000..a7fd02d0b
--- /dev/null
+++ b/changelog.d/1020.change.md
@@ -0,0 +1 @@
+Pickling and copying a slotted class that inherits a `__dict__` from a non-slotted base class now preserves attributes stored in the instance `__dict__` -- for example attributes set in `__attrs_post_init__` or by a mixin -- instead of silently dropping them.
diff --git a/src/attr/_make.py b/src/attr/_make.py
index e5b3760c1..44689c0f5 100644
--- a/src/attr/_make.py
+++ b/src/attr/_make.py
@@ -1035,7 +1035,20 @@ def slots_getstate(self):
             """
             Automatically created by attrs.
             """
-            return {name: getattr(self, name) for name in state_attr_names}
+            state = {name: getattr(self, name) for name in state_attr_names}
+
+            # A slotted class can inherit a __dict__ from a non-slotted
+            # base class. Such instances may carry additional state in
+            # their __dict__ -- for example attributes set by a mixin or
+            # in __attrs_post_init__ -- which is not managed by attrs.
+            # Preserve it like object.__getstate__() does, so pickling
+            # and copying do not silently drop it.
+            inst_dict = getattr(self, "__dict__", None)
+            if inst_dict:
+                for name, value in inst_dict.items():
+                    state.setdefault(name, value)
+
+            return state
 
         hash_caching_enabled = self._cache_hash
 
@@ -1054,6 +1067,17 @@ def slots_setstate(self, state):
                     if name in state:
                         __bound_setattr(name, state[name])
 
+                # Restore additional __dict__ state that was preserved by
+                # slots_getstate (see above). Instances without a
+                # __dict__ cannot hold such state; extra keys are ignored
+                # for them, which keeps unpickling working if an
+                # attribute was removed from the class.
+                inst_dict = getattr(self, "__dict__", None)
+                if inst_dict is not None:
+                    for name, value in state.items():
+                        if name not in state_attr_names:
+                            inst_dict[name] = value
+
             # The hash code cache is not included when the object is
             # serialized, but it still needs to be initialized to None to
             # indicate that the first call to __hash__ should be a cache
diff --git a/tests/test_slots.py b/tests/test_slots.py
index a74c32b03..de376b84f 100644
--- a/tests/test_slots.py
+++ b/tests/test_slots.py
@@ -4,6 +4,7 @@
 Unit tests for slots-related functionality.
 """
 
+import copy
 import functools
 import pickle
 import weakref
@@ -622,6 +623,19 @@ def f(self, a):
     C(field=1)
 
 
+@attr.s(slots=False)
+class CNonSlottedBase:
+    name = attr.ib()
+
+
+@attr.s(slots=True)
+class CSlottedChildWithDict(CNonSlottedBase):
+    x = attr.ib()
+
+    def __attrs_post_init__(self):
+        self.z = self.x + 2
+
+
 @attr.s(getstate_setstate=True)
 class C2:
     x = attr.ib()
@@ -645,6 +659,34 @@ def test_pickleable_by_default(self, protocol):
         assert i1 == i2
         assert i1 is not i2
 
+    @pytest.mark.parametrize("protocol", range(pickle.HIGHEST_PROTOCOL))
+    def test_pickle_preserves_dict_state(self, protocol):
+        """
+        Attributes stored in the instance __dict__ of a slotted class
+        that inherits a __dict__ from a non-slotted base class are
+        preserved when pickling, instead of being silently dropped.
+        """
+        i1 = CSlottedChildWithDict("spam", 3)
+
+        i2 = pickle.loads(pickle.dumps(i1, protocol))
+
+        assert i1 == i2
+        assert "spam" == i2.name
+        assert 3 == i2.x
+        assert 5 == i2.z
+
+    def test_copy_preserves_dict_state(self):
+        """
+        Copying a slotted class with a __dict__ (see above) preserves
+        the attributes stored in the __dict__ as well.
+        """
+        i1 = CSlottedChildWithDict("spam", 3)
+
+        i2 = copy.copy(i1)
+
+        assert i1 == i2
+        assert 5 == i2.z
+
     def test_no_getstate_setstate_for_dict_classes(self):
         """
         As long as getstate_setstate is None, nothing is done to dict