Morphometric quantitative analysis of the renal arterioles (arteriole wall thickness and overall vessel size) in renal biopsies from 21 cadaveric renal allograft recipients taken immediately prior to (0 months) and 3 months after transplantation was carried out using a computer-aided image analyzer. Patients (n = 10) who received standard therapy (cyclosporine, azathioprine and prednisolone) showed an increase in arteriole wall thickness at 3 months compared to 0 months (p < 0.01). Patients (n = 11) who received standard therapy and oral diltiazem (60 mg t.d.s.) showed no such change in the arteriole wall thickness between 0 and 3 months. As cyclosporine toxicity and/or chronic vascular rejection are associated with an increase in arteriole wall thickness, the differences observed in this study may imply a protective role for diltiazem on renal arterioles.