Human endometrial stromal cells (decidual cells) display dramatic alterations in cell shape and size during decidualization. The present study was designed to demonstrate the expression of two major cytoskeletal elements, desmin and vimentin, in human pregnant endometrial decidual cells. Additionally, stage-dependent variations of those intermediate filaments (IFs) among gestational weeks were also evaluated with regard to the support and maintenance of decidualization. Materials were obtained from legal suction terminations of pregnancies of 3-10 weeks gestation. Tissue specimens were either blocked in paraffin or enzymatically dissociated for isolation of decidual cells which subsequently were cultured as monolayers. Immunoperoxidase and immunofluorescence staining methods were applied by using anti-desmin and anti-vimentin monoclonal antibodies. Both desmin and vimentin expression were observed during the early weeks of pregnancy (3-6 weeks). These two types of IFs were also detected in short-term cultures in a filamentous fashion either within the cell body or at cellular attachment plaques. When decidual cells were cultured for longer periods (40-60 days), the expression of desmin dramatically declined while vimentin expression was maintained in a rather diffuse and more abundant fashion. The in situ expression of desmin and vimentin in later weeks of gestation (7-10 weeks) correlated with immunofluorescence staining of long-term cultured cells in that desmin staining was very weak and mostly undetectable where vimentin expression persisted and was evenly distributed throughout the entire stroma. The results demonstrate the differential expression of two major IFs, desmin and vimentin, in human endometrial stromal cells during decidualization and subsequent placentation. The persistence of vimentin in all stages examined suggests that this IF is probably involved in cell morphology and nucleocytoplasmic integrity. The temporal pattern of desmin expression suggests a role for this IF during the rapid onset of the decidualization process.