Hepatic acetylator phenotype has been determined, using sulfamethazine, in 81 white Spanish women with histologically proven breast cancer and in 75 adequate female controls. No differences were detected in the distribution of acetylator phenotype between the two groups of slow acetylators, 49 patients (60.5%) and 45 controls (60%). The percentage of acetylated sulfamethazine in plasma for each phenotype was not different either. Our results suggest that there is no relationship between the hepatic acetylator polymorphism and the risk of developing breast cancer in women.