Nitric oxide is generated by the NO synthases, a family of isoenzymes expressed in a wide range of mammalian cells. In the vascular and nervous systems distinct isoforms generate NO to act as a signal transduction mechanism. The isoform induced by cytokines, on the other hand, provides a sustained release of NO which mediates some cytotoxic and cytostatic effects of the immune system. Solid tumors are a heterogeneous population of cell types, including tumor, vascular, and infiltrating immune cells. Studies in vitro show that NO synthase can be present in many of these cells. However, its presence in situ in solid human tumors has not been reported. In this study, we have investigated NO synthase activity and its cellular localization in malignant and nonmalignant human gynecological tissue. Nitric oxide synthase activity was observed in malignant tissue, was highest (≥250 pmol/min/g tissue) in poorly differentiated tumors, and was below detectable levels in normal gynecological tissue. Furthermore, investigations with a polyclonal NO synthase antibody revealed immunoreactivity only in malignant tissue. This was associated with NO synthase activity and localized to tumor cells. Thus NO synthase is present in human gynecological tumors, and its presence seems to correlate inversely with the differentiation of the tumor.