All 44 neurosurgical units in the British Isles replied to a postal questionnaire asking about their use of neurosurgery during 1974--6 for functional mental illness. A total of 431 operations was reported, representing a yearly rate of 3.4 operations per million population aged over 15. The numbers of operations declined from 158 in 1974 to 119 in 1976. Four units did two-thirds of the operations. Stereotactic methods for locating the site for the lesion were used in two-thirds of procedures. Mood disorders, anxiety states, and obsessive-compulsive neurosis were the conditions most commonly treated.