CASE PRESENTATION: A previously healthy 31-year-old man presented with a two-week history of left eye (OS) ptosis and diplopia, following botulinum toxin injection over the masseter area for masseter hypertrophy at an aesthetic centre. He had no proptosis or facial asymmetry. Visual acuity was 6/6 in the right eye (OD) and 6/9 in the OS. There was anisocoria, with pupils measuring 3 mm in the OD and 5 mm in the OS but no relative afferent pupillary defect. OS appeared hypertropic in primary gaze with impaired intorsion. Extraocular movement of the OS was restricted in all gazes, except for laevoversion; that of the OD was normal. This was associated with diplopia in all gazes except on laevoversion. Both eyes' anterior and posterior segment examinations were otherwise unremarkable. Besides the oculomotor and trochlear nerve, the other cranial nerves and neurological examinations were normal. Investigations including blood and cerebrospinal fluid, magnetic resonance imaging and angiography of the brain, were normal. Our impression was left oculomotor and trochlear nerve neuritis secondary to botulinum toxin injection. He was started on oral prednisolone 1 mg/kg daily and tapered by 5 mg per week. His condition improved gradually with no residual ptosis or anisocoria after three months. Extraocular movements normalised except for minimal residual restriction on depression.