Polymorphic alleles of CYP17 and CYP19, which are involved in estrogen biosynthesis, were tested for association with breast cancer (BC). Microsatellite (TTTA)n and 3-bp deletion of CYP19 and single-nucleotide polymorphism T27C of CYP17 were analyzed in 123 BC patients and 119 healthy women. Of the six (TTTA)n alleles observed, allele (TTTA)8 proved to be associated with BC (11.8% vs. 6.3%, P = 0.04). Genotype A2/A2 of CYP17 was also associated with BC (32.5% vs. 20.2%, P = 0.04). Risk of BC was especially high in the presence of both factors (7.3% vs. 0%, P < 0.01). Allele (TTTA)8 and genotype A2/A2 were assumed to be risk factors of BC.