Of 113 cyclosporine-treated primary renal allograft recipients, 60 were randomized to receive standard therapy without diltiazem (ND) and 53 received standard therapy plus diltiazem (D). There was no difference in CsA blood levels between ND and D at all intervals between 3 and 24 months follow-up, yet the D group required 35% less CsA than the ND group (measured at 12 months). At all intervals to 24 months there was no difference in blood pressure, renal function (as measured by serum creatinine), or in the number of grafts lost between the 2 groups (ND, 4 lost; D, 3 lost). There was no significant difference in the total number of rejection episodes in the 2 groups (ND, 89 episodes; D, 71 episodes). However, the severity of rejection episodes was greater in the ND group as evidenced by a significant difference in the usage of OKT3 (ND, 17 courses; D, 8 courses of OKT3, P < 0.05). Of the biopsy-proven episodes of rejection, there were more episodes of vascular rejection in the ND group (ND, 14 episodes; D, 3 episodes, P = 0.005). The incidence of primary nonfunction was less in the D group (ND, 16 patients; D, 5 patients, P = 0.05). It was concluded that the use of diltiazem was associated with a markedly reduced requirement for CsA without any adverse effect on graft function or graft outcome. Diltiazem with CsA was associated with fewer episodes of primary nonfunction and less-severe rejection episodes and in particular fewer episodes of vascular rejection.