During a clinical trial of duration of chemotherapy in small cell lung cancer (SCLC), 71 of 610 patients (11.6%) died in the first 3 weeks. Chemotherapy consisted of cyclophosphamide 1 g m-2 i.v. day 1, etoposide 100 mg t.d.s. orally days 1-3, vincristine 2 mg i.v. day 1. The time of death was found to be nonrandomly distributed within the first chemotherapy cycle, with a peak incidence between days 7 and 12 after chemotherapy. Patients were matched with controls who were the next cases entered into the study who did not die in the first 3 weeks. Patients dying early were more likely to have clinical hepatomegaly (P less than 0.0001), and ECOG score greater than or equal to 1 (P less than 0.00001). As a group these patients also had a higher alkaline phosphatase (P less than 0.0002), an elevated blood urea (P less than 0.00001) and a lower serum albumin (P less than 0.0001) than controls. It is probable that infection contributes to the death of these already ill patients at a time when the blood count is low. Early deaths have been noted in two other large trials using regimens including etoposide. Prophylactic antibiotics or dosage modification may prevent the early death of these high risk patients.