Thirty-four patients with adrenocortical carcinoma were treated with o,p′-DDD. Twenty-eight patients presented with metastases at entry, and spillage of tumour cells occurred at surgery in 6 other patients. Eight patients had objective tumour regression, of whom 7 had serum levels over 14 μg/ml. The 3 patients with a lasting remission had levels of >15, >25, >25 μg/ml respectively during prolonged periods. Increased survival times were found in the group of 14 patients with o,p′-DDD serum levels higher than 14 μg/ml when compared with patients not treated after discovery of metastases. In the patients with levels ⩽10 μg/ml no therapeutic effect was seen. Levels of over 20 μg/ml are associated with symptoms of reversible neuromuscular toxicity. Monitoring of serum levels during treatment is mandatory. It is suggested that serum levels of about 25 μg/ml during longer periods may be curative.