Smooth muscle antibodies (SMA) were monitored in terms of titer, immunoglobulin class and staining pattern by indirect immunofluorescence assay during the pregnancies of 32 women with pregnancy induced hypertension (PIH), 21 with essential hypertension (EH), and 43 with a normal pregnancy. The mean SMA titer was higher both in PIH and EH than in normal pregnancy. SMA were mainly of the IgG class and gave a "vessel wall" staining pattern in immunofluorescence. The tendency for the mean SMA titer to increase was observed in PIH, whereas a falling tendency was found in EH and normal pregnancy. These tendencies suggest that hypertension, the duration of which has been shorter in PIH, induces SMA production. The observed staining pattern suggests the same kind of non-actin specificity for SMA as in our previous study of complications in early pregnancy.