BACKGROUND Superior oblique palsy is accompanied in most cases by overaction of the muscle's ipsilateral antagonist, the inferior oblique muscle. Overaction and contracture of the ipsilateral superior rectus muscle in patients with unilateral fourth (trochlear) nerve palsy is seldom discussed in the literature. The purpose of this study is to evaluate the frequency of superior rectus muscle overaction/contracture syndrome in patients with unilateral trochlear nerve palsy. METHODS The records of 198 patients with unilateral trochlear nerve palsy examined by the authors between July 1987 and July 2008 were reviewed retrospectively. All patients underwent complete eye examination with measurement of the deviation in the 9 positions of gaze and with the head tilted to both sides in all cooperative patients. Selection criteria for superior rectus muscle overaction/contracture syndrome in these patients were as follows: vertical deviation of 15(Delta) or larger in primary position, equal or larger hypertropia with the ipsilateral forced tilt test than with the eyes looking straight ahead, more than 5(Delta) hypertropia of the affected eye in horizontal gaze to the same side, hypertropia in all upgazes, and overaction of the contralateral superior oblique muscle. RESULTS Of 198 patients, 33 (16.6%) met the selection criteria for superior rectus muscle overaction/contracture syndrome. CONCLUSIONS Superior rectus muscle overaction/contracture syndrome frequently occurs in unilateral superior oblique palsy.