Carcinoma of the oesophagus is endemic in certain well demarcated areas throughout the world, and a method of screening population groups at high risk for oesophageal cancer is urgently needed. In this study the sensitivity and specificity of the carbohydrate antigen CA19-9 as a marker of carcinoma of the oesophagus in African patients was examined. The normal range was established by assay of serum samples from healthy black blood donors, using a solid phase radioimmunoassay with mouse monoclonal antibody to CA19-9 labelled with 125I. Serum concentrations of CA19-9 were then measured in 100 African patients with oesophageal cancer and 28 patients with benign oesophageal disease. The upper limit of CA19-9 in the normal controls was 40 U ml-1. Thirty-four patients with oesophageal cancer and five with benign oesophageal disease had elevated levels. Therefore, in this series, the sensitivity of CA19-9 as a marker of oesophageal cancer was 34% and the specificity was 82%. While CA19-9 is not sufficiently sensitive to be used as a screening test of oesophageal cancer, it compares favourably with other known tumour markers of this disease, and may have a role in monitoring disease recurrence and response to treatment.