OBJECTIVE This study employed magnetic resonance imaging (MRI) to investigate possible size and contractility changes in the superior oblique (SO) muscle, and possible isometric hypertrophy in the inferior oblique (IO) muscle, resulting from abnormal mechanical loading in Brown syndrome (BrS). METHODS High resolution orbital MRI was obtained in 4 congenital and 11 acquired cases of BrS, and compared with 44 normal subjects. Maximal cross-section areas and posterior partial volumes (PPVs) of the SO were analyzed in central gaze, supraduction, and infraduction [corrected] for the SO, and in central gaze only for the IO. RESULTS In congenital BrS, mean maximum SO cross-sectional areas were 24% and 20% less than normal in affected and unaffected eyes, respectively (P = 0.0002). Mean PPV in congenital BrS was also significantly subnormal bilaterally (29% and 34% less in affected and unaffected eyes, respectively, P = 0.001). However, SO muscle size and volume were normal in acquired cases. The SO muscle did not relax in supraduction in BrS, although there was normal contractile thickening in infraduction. The IO muscle had normal size bilaterally in BrS. CONCLUSIONS Congenital BrS may be associated with SO hypoplasia that could reflect hypoinnervation. However, unique isometric loading of oblique extraocular muscles due to restrictive hypotropia in adduction in BrS is generally not associated with changes in muscle bulk or in SO contractility. Unlike skeletal muscles, the bulk and contractility of extraocular muscles can therefore be regarded as independent of isometric exercise history. Restriction to elevation in BrS typically arises in the trochlea-tendon complex.