The authors reviewed the medical records of 1005 patients with multiple colorectal cancers (763 synchronous and 242 metachronous) to study the number and site of the tumors, their preoperative detectability, and the results of treatment. Concurrent advanced cancers were found in 35.1%, concurrent advanced and early cancers in 59.0%, and concurrent early cancers in 5.9% of patients with synchronous malignancy. In 60.1% of these patients, the existence of multiple lesions was diagnosed preoperatively; coexisting early cancers were often overlooked. In patients with metachronous malignancy, early cancers were less frequent than in patients with synchronous malignancy. The cumulative 5-year survival rate in curatively operated patients was 70.4% for synchronous malignancy and 66.5% for metachronous malignancy, similar to that for colon cancer in general.