The goal of this study was to investigate the expression of these isoforms in different regions of the human uterus. Expression was determined in the endometrium (ENDO), endocervix (CERV) and squamous region (SR) from six non-smoking women by using RT-PCR. The transcripts encoding for CYP1A1 were significantly higher (P < 0.05) in the SR compared to the other areas. However, the expression of CYP1B1 was significantly higher (P < 0.05) in the ENDO. CYP1B1 expression appeared to be extremely low in a woman in the secretory stage of the menstrual phase, relative to the ENDO of the other patients who were all in the proliferative stage at hysterectomy. CYP1B1 mRNA was expressed in only two out of six patients in the SR and in three out of five patients in the CERV. CYP1A1 was also uniformly expressed in the ENDO of all except one patient, whereas expression was minimal in the other regions. It is likely that variability in the expression of these isoforms may be responsible for the differential susceptibility to cancer in women.