The clinical course of colorectal carcinoma may be monitored by tumor markers such as carcinoembryonic antigen (CEA), carcinoma antigen (CA) 19-9 and CA-50. Alkaline phosphatase isozymes were previously used to study the clinical course of testicular and gynecologic tumors. In this study we investigated 8 patients with advanced colorectal carcinoma. Their sera were analyzed for the tumor markers CEA, CA 19-9, CA-50 and three alkaline phosphatase isozymes: the nonspecific liver isozyme LAP, the intestinal isozyme IAP and the placental isozyme PLAP. Rising levels of CEA, CA 19-9 and CA-50 were seen as expected, and PLAP also showed rising levels during tumor progression. LAP remained elevated. This indicates an association between progression of colorectal carcinoma and a raised serum content of alkaline phosphatase isozymes.