We studied samples from gallbladders of 10 subjects with invasive and 10 with in situ carcinoma of this organ. The number of nucleolar organizer regions, as revealed by colloidal silver staining, was at least 4 times larger in cancer samples than in control ones. Mean number of nucleolar organizer regions for invasive lesions was higher than for in situ ones (11.6 +/- 2.3 vs 9.4 +/- 1.3, p less than 0.04). The study of cellular markers may be helpful in establishing the preneoplastic nature of gallbladder lesions.