Abstract A xenobiotic-metabolizing enzyme, the P450IA1 (aryl hydrocarbon hydroxylase; AHH) was measured fluorometrically in myometrium and in uterine leiomyoma tissues. The material included both myometrium and leiomyoma tissue samples from 62 leiomyoma patients and 22 tissue samples of normal myometrium obtained from leiomyoma-free patients undergoing hysterectomy. Among 62 leiomyoma patients there were 14 cigarette-smokers and 48 non-smokers, and among 22 control patients 6 and 16, respectively. The P450IA1 activity of the leiomyomas was significantly ( P < 0.0005) higher than that of the surrounding myometrium in both smoker and non-smoker groups. In smokers, myometrium an obvious induction of P450IA1 was seen. Smoking increased the P450IA1 activity also in the leiomyoma, but the increased was not statistically significant. Our study supports the idea that myometrial tissue can respond to xenobiotics from the environment.