An alcoholic woman who was admitted to hospital for detoxification was prescribed thyroxine because of hypothyroidism and mianserin to alleviate severe depression. After several weeks' treatment she became unwell and was readmitted to hospital. Haematological examination indicated agranulocytosis. Further extensive investigations elicited no cause for this other than the mianserin, since no such disturbance has been reported for thyroxine after years of use. Thus mianserin is probably implicated in this case of agranulocytosis. Although the response may have been idiosyncratic, it highlights the need to monitor new drugs during the early phases of widespread use.