T h e determination of a paretic vertical muscle may be made by prism cover test measurements in the 9 diagnostic positions of gaze or the 3-step test which relies on the Bielschowsky head tilt test. A series of cases with an isolated paretic vertical muscle was used to compare the 2 methods of examination. It was found that 1) the 3-step test (based o n the Bielschowsky head tilt test) is non-diagnostic as frequently as fielding out, 2) the 3-step test is much more likely to diagnose an oblique (especially the superior oblique) than any other muscle, 3) there is a statistically significant association between a loger duration of the paresis and the appearance of spread of comitance, and 4) the disagreement between the 3-step test and cardinal gaze measurements which occurs in 18% of cases can not be adequately explained by 'overspread of comitance' to the point where the deviation is maximum in the opposite field of gaze. T h e measurement of the deviation in the cardinal positions of gaze was felt to more accurately indicate the paretic muscle when the two tests were in conflict.K q ulord,s: Bielschowsky head tilt test -overspread of comitance -the 3-step test -vertical muscle palsy.The problem of diagnosing a paretic ocular cyclovertical muscle has always been difficult. As early as 1889, Duane (1889, 1914, 1919) popularized the 'screen test' because of its accuracy and convenience. The modern equivalent consists of the prism cover test in the 9 cardinal positions of gaze. The position of maximum vertical deviation indicates the paretic muscle.