Over the past decade various clinical trials have used monoclonal antibodies as therapeutic agents against solid tumours. No consistent pattern of response or improved survival has yet emerged although antigenic heterogeneity and insufficient accessibility of cells in advanced tumours have been offered as explanations for these failures. We designed a study in which a monoclonal antibody was used to target minimal residual disease in an early stage of tumour cell dissemination in patients with colorectal cancer. Only patients in Dukes' stage C who had undergone curative surgery and were free of manifest residual tumour were admitted. 189 patients with colorectal cancer of stage Dukes' C were randomly assigned to an observation regimen or to postoperative treatment with 500 mg of 17-1A antibody, followed by four 100 mg infusions each month. A balance of risk factors in the two groups was achieved by dynamic randomisation procedure. After a median follow-up of 5 years, antibody treatment reduced the overall death rate by 30% (Cox's proportional hazard, p = 0.04, log-rank p = 0.05) and decreased the recurrence rate by 27% (p = 0.03, p = 0.05). The effect of antibody was most pronounced in patients who had distant metastasis as first sign of a relapse (p = 0.0014, p = 0.002), an effect that was not seen for local relapses (p = 0.74, p = 0.67). Toxic effects of 17-1A antibody were infrequent, consisting mainly of mild constitutional and gastrointestinal symptoms. During 371 infusions four anaphylactic reactions were seen, all controllable by intravenous steroids and none necessitated admission to hospital. Adjuvant therapy with 17-1A antibody extends life and prolongs remission in patients with colorectal cancer of Dukes' stage C.