Multiple synchronous colorectal cancer (MSCC) among 1537 patients (69 with familial polyposis coli (FPC), 780 with ulcerative colitis (UC), and 685 with de novo colorectal (DNC) cancers) admitted to The Mount Sinai Hospital between 1945 and 1981 was tabulated. MSCC occurred in five of 24 cancer patients with FPC (21%), in 12 of 65 cancer patients with UC (18%), but in only 17 of 685 DNC patients (2.5%). The proportions of MSCC cases with more than two synchronous tumors were also much greater in the former two groups (UC 6/12 = 50%, FPC 3/5 = 60%) than in DNC (0/17 = 0%). Multiplicity of cancers is thus a distinguishing feature of UC and FPC. MSCC differed from solitary cancers by association with older age and more advanced stage at diagnosis in patients with FPC and by a rightward shift in anatomic distribution in all patients, especially those with FPC and UC.