Familial adenomatous polyposis coli (FAP) has been shown to be associated with germline mutations of the adenomatous polyposis gene (APC) on chromosome 5. Extra-colonic manifestations also occur in FAP and include desmoid tumors, epidermoid cysts and osteomas. The combination of FAP with extracolonic symptoms is commonly referred to as Gardner's syndrome. It remains difficult, however, to predict which patients may have a propensity to develop extracolonic manifestations. The rapid acetylation phenotype is believed to be associated with an increased likelihood of sporadic colorectal cancer, whereas the slow acetylation phenotype is recognized as a predisposing factor for bladder cancer. The slow acetylation phenotype is caused by mutant alleles of the cytosolic enzyme N-acetyltransferase (NAT2). In this study, we determined the NAT2 genotype in members of one large FAP family and three smaller ones all of which had been shown to harbor the same germline APC gene mutation. We observed a significant correlation between slow acetylation genotypes and extracolonic manifestations of the disease. Rapid acetylation genotypes were not overrepresented in colorectal cancer cases in this family as compared to the frequency of this genotype in the normal Caucasian population.