The incidence of colorectal carcinoma was compared with the incidence of ABO and Lewis blood groups. The raw data showed the known overrepresentation of the Le(a-b) phenotype, but also suggested an association of colorectal carcinoma with the Le(a-b+) phenotype in group O individuals. When the data were adjusted by taking into account the known loss of Lewis antigens by Lewis-positive patients, this association could be shown to be statistically significant. These results may indicate involvement of the secretory H antigen in colorectal carcinoma.