There are currently no effective, long-term drug therapies for the treatment of leiomyomas. Pirfenidone (Marnac, Inc.) is an antifibrotic agent that is being tested for use in patients with pulmonary fibrosis. Because leiomyomas are characterized also by increased cell proliferation and tissue fibrosis, we examined the effects of pirfenidone on cell proliferation and collagen expression in cultured myometrial and leiomyoma smooth muscle cells. Effects of pirfenidone on proliferation of myometrial and leiomyoma cells were measured using tritiated thymidine incorporation assays and changes in actual cell numbers. Possible cytotoxic effects were examined using lactate dehydrogenase assays and trypan blue exclusion. Effects on collagen type I and type III production were assessed by Northern blotting. Doses of pirfenidone tested were: 0, 0.01, 0.1, 0.3, and 1.0 mg/mL. Serum-stimulated increases in DNA synthesis and cell proliferation by myometrial and leiomyoma cells were significantly inhibited in a dose-dependent manner by pirfenidone. Densitometric analysis of Northern blots showed significantly decreased expression of collagen type I and type III messenger RNAs in both leiomyoma and myometrial cells. Lactate dehydrogenase assays and trypan blue exclusion measurements showed no cytotoxic effect of pirfenidone at concentrations that inhibited cell proliferation and collagen production. Pirfenidone is an effective inhibitor of myometrial and leiomyoma cell proliferation in vitro and reduces the messenger RNA levels of collagen types I and III in a dose-dependent manner. This compound may prove to be an effective nonsteroidal therapy for treatment of uterine leiomyomas.