The MspAI polymorphism in the 5' untranslated region of CYP17 has been evaluated as a breast cancer risk factor in a hospital-based case-control study in New York City. The study population consisted of 363 women [123 breast cancer patients and 240 patient controls (123 benign breast disease without atypical hyperplasia, 117 women without breast disease)]. There were 224 Caucasians (76 cases, 148 controls), 55 African-Americans (20 cases, 35 controls) and 84 Hispanics (27 cases, 57 controls); 142 premenopausal women and 221 postmenopausal women. Consistent with a previous report (Feigelson et al., Cancer Res., 57: 1063-1065, 1997) we found no evidence to implicate the minor variant (restriction site present allele, designated A2) as a breast cancer risk factor. Furthermore, we sought evidence to implicate the minor variant of CYP17 in the development of more aggressive breast cancers (n = 38/121) as had been reported previously. Although confidence intervals (CI) overlap, the data presented here do not provide support for previously reported findings (odds ratio, 0.9; 95% CI, 0.4-2.0; n = 38 versus odds ratio, 2.5; 95% CI, 1.1-5.2; n = 40). Clearly this question needs to be resolved in a larger study. No evidence was found to support the contention that inheritance of the minor variant is a predictor of early age at menarche. Allelic frequencies between different ethnic groups were not found to be different with the exception of Hispanic controls, in which the genotypic distribution was not consistent with the Hardy-Weinberg equilibrium.