Our understanding of the effects of midbrain damage on cognition is largely based on animal studies, though there have been occasional investigations of the effects of human midbrain lesions on cognition. This investigation of a rare case of a glioma initially confined to the dorsal midbrain explores the effects of disease progression on IQ, memory, and choice reaction time. Extensive dorsal midbrain damage did not appear to affect IQ and various memory functions (including span, working memory, story recall, and remote memory). Choice reaction time latencies increased, while verbal and spatial learning and long-term memory deteriorated with tumour growth, but it was not clear how far the deterioration reflected midbrain damage or damage outside the midbrain. (Contains 2 tables and 7 figures.)