diff --git a/fsl/data/image.py b/fsl/data/image.py
index 46fb47f813b3dbca90888a45a3a01245f635a350..b65f28105668b76867f41f6d3f0c7a0fa8100c44 100644
--- a/fsl/data/image.py
+++ b/fsl/data/image.py
@@ -1362,6 +1362,10 @@ class Image(Nifti):
     def save(self, filename=None):
         """Saves this ``Image`` to the specifed file, or the :attr:`dataSource`
         if ``filename`` is ``None``.
+
+        Note that calling ``save`` on an image with modified data will cause
+        the entire image data to be loaded into memory if it has not already
+        been loaded.
         """
 
         import fsl.utils.imcp as imcp