This study was performed to clarify the relationship between changes in contractile proteins in renal vascular walls and the prognosis of hypertension during pregnancy. Twenty preeclamptic patients underwent renal biopsies after delivery and were divided into the following three groups: group I, patients with persistent hypertension after delivery (n = 7; mean age, 34.8 +/- 1.4 years [SE]); group II, patients who became normotensive after delivery and hypertensive again during follow-up (n = 5; mean age, 34.8 +/- 1.6 years), and group III, patients who became normotensive after delivery (n = 8; mean age, 28.0 +/- 1.0 years). We also examined age-matched healthy controls (group IV; n = 7; mean age, 34.9 +/- 1.5 years). Renal biopsy specimens were immunohistochemically stained by the avidin-biotinylated peroxidase complex method using antimonoclonal smooth muscle cell myosin heavy chain isoform antibodies (SM-1, SM-2) and antimonoclonal alpha-smooth muscle cell actin antibody (actin). We estimated and semiquantitatively scored the degree of staining in each section. In interlobular arteries, SM-1, SM-2, and actin staining in group I were significantly reduced compared with group IV (SM-1, SM-2, P: < 0.05; actin, P: < 0.01). In afferent arterioles (Afs), SM-1, SM-2, and actin staining were reduced in group I. SM-2 staining in group I was significantly reduced compared with the other three groups (versus group II, P: < 0.05; versus groups III and IV, P: < 0.01). These findings suggest that phenotypic changes in vascular smooth muscle cells (especially the disappearance of SM-2 in Afs) reflect the stage of underlying essential hypertension and can predict from the change in hypertension during pregnancy whether it will persist after delivery.