BACKGROUND: Our aim was to describe, in a population-based setting, the use of colonoscopy over time with special emphasis on indications, the competence of the endoscopists, and the frequency of total colonoscopies and to compare the number of colonoscopies with that of barium enemas. METHODS: All colonoscopy records during 1979-95 in a Swedish county (population, 258,000) were retrieved. Information was collected about the patients' demographics, the endoscopists, indications, findings, and type of colonoscopy. RESULTS: A total of 6066 colonoscopies were performed in 4304 patients by 62 endoscopists. Of these, 562 were not intended to be complete; of the other 5504 colonoscopies, 4153 were complete (75%). Of the patients 77% had undergone colonoscopy once, 14% twice, and 9% three times or more. Among the endoscopists 73% never performed more than 50 colonoscopies, and 5% did more than 700 colonoscopies. Bleeding as an indication increased from 10% to 31%; polyps decreased from 30% to 10%, and unclear X-ray findings decreased from 28% to 4%. Diarrhoea and abdominal pain, both 0% in 1979, increased to 6% and 5%, respectively. Surveillance (cancer, polyps, colitis) was fairly stable, at 25%. Both the rate of complete colonoscopies and the proportion of colonoscopies done by experienced endoscopists increased with time. The annual number of barium enemas was relatively constant until 1992 but then decreased. CONCLUSIONS: The increased use of colonoscopy has resulted in a decrease in barium enemas. The competence of the endoscopists increased, resulting in a higher rate of complete colonoscopies.