Ninety-two patients treated surgically between 1972 and 1983 were diagnosed initially as having a unilateral superior oblique palsy (SOP). On follow-up, eight (8.7%) developed findings of an SOP of the other eye. Such patients have "masked bilateral" superior oblique palsy. All eight patients had unilateral inferior oblique weakening with or without contralateral inferior rectus recession as the initial surgery. The "unmasking" of the contralateral palsy manifested in seven cases by an overaction of the contralateral inferior oblique muscle and appeared an average of 9.8 weeks (range five days to 11 months) after the first surgery. Their preoperative mean V-shift (7.6 diopters), mean excyclotorsion (5.7 degrees), and mean difference of hyperdeviation on right versus left tilt (19.6 diopters) were not significantly different from the respective values for patients with unilateral SOP (5.0 diopters, 4.3 degrees, 19.0 diopters).