In order to clarify the relation of adenoma to carcinoma in the gallbladder, histopathologic examination was made on surgical specimens of 1605 cholecystectomies. Among them, 11 benign adenomas, seven adenomas with malignant change, and 79 invasive carcinomas were found. All of the benign adenomas were 12 mm or less in diameter (average diameter, 5.5 +/- 3.1 mm), while the adenomas having cancerous foci were 12 mm or more in diameter (average diameter, 17.6 +/- 4.4 mm). Most invasive carcinomas were more than 30 mm in diameter. The average patient age was 50.5 +/- 16.3 years for benign adenomas, 58.3 +/- 12.6 years for adenomas with malignant change, and 64.8 +/- 9.6 years for invasive carcinomas. Transition of benign adenoma into carcinoma was histologically traceable. Adenomatous residue was found in 15 (19.0%) of 79 cases of invasive carcinoma.