BACKGROUND: An extraocular muscle palsy is conventionally characterized by a deviation of the visual axes, this being greater when measured with the affected eye fixing. By definition and illustrating Hering's law, this secondary angle of deviation is greater than the primary one, measured with the sound eye fixing. We present here a comparative study of the amount of subjective excyclodeviation measured in patients suffering from IVth nerve palsy, with the sound or affected eye fixing. METHODS: Two groups of patients were entered into the study: Group 1 (N = 54) for superior oblique palsies studied retrospectively and Group 2 (N = 14), for a prospective study of those recently acquired (post-traumatic) and followed over 6 months. In both groups, measurements were made at two stages, early (1 to 7 weeks after onset) and late (4 to 6 months later). RESULTS: In both groups, the majority of cases showed a greater secondary torsional deviation, the difference between this and the primary deviation lessening on late stage measurements. CONCLUSION: In both groups, the difference between primary and secondary torsional deviation was not statistically significant.