The development of metachronous adenomas was evaluated in 56 patients, 34 men and 22 women, with a history of 139 removed adenomas and non-neoplastic polyps, in order to determine whether the presence of concomitant adenomas and hyperplastic polyps at the initial examination could predict a higher risk of new adenomas. Sixteen of the 56 patients developed 21 metachronous adenomas and 1 rectal carcinoma after a median follow-up of 2.8 years. Among the 16 patients developing metachronous adenomas there were significantly more men (P less than 0.01), patients with multiple adenomas (P less than 0.05) or patients with adenomas and synchronous hyperplastic polyps at the initial examination (P = 0.043) than among the 40 patients without new adenomas. The combination of multiple adenomas with synchronous hyperplastic polyps at the initial examination predicted best the risk of developing new adenomas (P = 0.005). The results of the study suggest that patients with multiple adenomas and synchronous hyperplastic polyps at the initial examination may comprise a new higher risk group for developing metachronous colorectal adenomas.