Fifty-six patients with Barrett's oesophagus diagnosed between 1977 and 1986 were prospectively studied by 6-monthly endoscopic surveillance and biopsy. During follow-up to-date, four patients have developed high-grade dysplasia and three have adenocarcinoma of the oesophagus. Two of the adenocarcinomas were preceded by progressively severe dysplastic changes but in the third no dysplasia had been previously detected. The incidence of adenocarcinoma was 1 per 56 patient-years of follow-up. Changes in symptomatology or gross endoscopic appearances were usually absent, even after adenocarcinoma had developed, indicating that biopsy is essential for early diagnosis. The high risk of malignant change makes endoscopic surveillance advisable in all patients with Barrett's oesophagus.