A 57-year-old woman developed features of Cushing's syndrome after resection of a Duke's C adenocarcinoma of the sigmoid colon. Biochemical and endocrine investigation indicated ectopic production of adrenocorticotrophic hormone (ACTH) as the cause for her condition. Hepatic metastases were detected by computed tomography (CT) scan. Histology of the original tumour displayed neuroendocrine characteristics but no definite evidence of ACTH synthesis. Treatment was instituted to control her hypercortisolism, and chemotherapy initiated to reduce the production of ectopic hormone. A clinical, biochemical and radiological response was obtained with complete resolution of her Cushing's syndrome. The tumour relapsed after several months with distant metastases, but no further endocrine abnormality was noted. A review of ectopic ACTH producing adenocarcinoma is given along with a discussion of the major pathological and therapeutic features of the case.