In an attempt to understand the pathogenesis of renal vein thrombosis occurring early after renal transplantation, gene expression of plasminogen activator inhibitor-1 (PAI-1) was investigated by an in-situ hybridization technique. The cases examined were six transplant kidneys complicated by renal vein thrombosis, four 'normal' kidneys and five time-matched transplant kidneys not complicated by renal vein thrombosis but showing acute tubular necrosis, infection, or normal histology. The cell types expressing PAI-1 mRNA were also studied by combined in-situ hybridization and immunohistochemical double staining techniques. Our results showed that PAI-1 mRNA was expressed in transplant kidneys complicated by renal vein thrombosis but there was no detectable expression in 'normal' kidneys, nor in time-matched transplant kidneys not complicated by thrombosis. Double staining showed that PAI-1 mRNA was predominantly expressed by capillary endothelial cells, particularly around large- or medium-sized renal arteries and small nerves. Smooth-muscle cells in the wall of major or medium-sized renal arteries also showed positive expression of PAI-1 in three of six thrombosed transplants. However, endothelium in the major renal vein showed relatively little signal. The pattern was different from that in rejection. The possible relevance of these findings is discussed.