This study examined the immunohistochemical findings in renal arterioles from biopsy specimens, and related the findings to those of light and electron microscopy. The renal biopsy specimens were obtained from 57 normotensive patients with primary glomerular diseases or idiopathic hematuria, 14 hypertensive patients with associated primary glomerular diseases, 4 patients with essential hypertension and 1 with primary aldosteronism. The tissue slices for the immunohistochemical study were processed with FITC-labelled rabbit antihuman immunoglobulin antisera. Deposits of IgM were detected on the renal arterioles in 16 of the hypertensive patients (84%), but in only 7 of the normotensive patients (12%). The difference in incidence was significant (p<0.005). C3 was almost always deposited in the renal arterioles regardless of whether the patient was hypertensive or normotensive. IgG, IgA or fibrinogen were demonstrated only in a few cases, and albumin in no cases. When sections stained with horseradish peroxidase-conjugated anti-IgM antibody were compared under a light microscope with the adjacent PAS-stained sections, it was demonstrated that IgM was deposited only in the portions of the arteriolar walls which showed hyalinotic changes. Electron microscopic examination demonstrated that electron-dense deposits in the subendothelial or intercellular spaces of arteriolar walls were more frequent in the hypertensive patients (11 of 14 cases, 79%) than in the normotensive patients (5 of 18 cases, 28%, p<0.05). The deposits appeared to be similar to those which are often found in the glomeruli of patients with glomerulonephritis and which are considered to be immune complexes. These findings suggest that some immunologic mechanism mediated by IgM antibody might be a factor in the development of hypertensive vascular lesions.