Median neuropathy is commonly associated with acromegaly, although its pathology is uncertain. To study the pathology of median neuropathy in acromegaly by using magnetic resonance imaging (MRI). Case series. Outpatient clinic and MRI unit. Nine patients with acromegaly, four of whom had clinical symptoms of neuropathy. At presentation and 6 months after treatment, median nerve size, its signal intensity, and the volume of the carpal tunnel contents were measured. At presentation, patients with symptoms of neuropathy had increased nerve size and signal intensity compared with asymptomatic patients, but the two groups did not differ in volume of carpal tunnel contents. These measures improved with treatment of acromegaly in symptomatic patients; asymptomatic patients experienced no change or worsening. The predominant pathology of median neuropathy in acromegaly seems to be increased edema of the median nerve within the carpal tunnel rather than extrinsic compression from increased volume of the carpal tunnel contents.