The expression of nitric oxide synthase (NOS) in human gynecological cancers, including ovarian cancers, uterocervical cancers, and endometrial cancers for example, was examined by the reverse transcriptase/polymerase chain reaction, coupled with Southern hybridization and by immunohistochemistry. Nitric oxide synthase II (NOS II), an inducible form, was expressed in more than 90% of the cancers. Nitric oxide synthase I (NOS I), a neuronal form, was expressed in 58% of all the ovarian cancers, in which the serous type is found more frequently (5 out of 7) than the mucinous type (2 out of 6), and in all clear-cell cancers. The frequency of NOS I expression in uterocervical cancers and endometrial cancers was relatively low. Nitric oxide synthase III (NOS III), an endothelial form, was detected in 25% of ovarian and 33% of endometrial cancers, while no expression was detected in uterocervical cancers. In terms of cancer types, all clear-cell adenocarcinomas and most of the serous-type adenocarcinomas expressed both NOS I and NOS II, while most uterine squamous carcinomas and endometrial adenocarcinomas expressed only NOS II. However, there was no correlation between the frequency of NOS expression and patients' age or the clinical stage of the disease. Since NO increases vascular permeability and blood flow, the high frequency of NOS expression in gynecological cancers may serve to stimulate and promote tumor growth.