To compare the red blood cell (RBC) deformability in hypertensive patients and healthy volunteers and to evaluate the effects of calcium antagonists on RBC deformability in hypertension, forty mild-to-moderate hypertensive patients and forty healthy volunteers, matched for age, sex and smoking habits, were studied. Each patient was randomized to receive one of the four following two-week treatment regimens: nifedipine 10 mg, verapamil 80 mg and diltiazem 60 mg three times a day and no drug. RBC deformability, measured with a gravity driven filtration technique, and mean arterial pressure were determined before and after the treatment. No difference in RBC deformability was observed between hypertensive patients and healthy volunteers. In contrast to the significant reduction in mean arterial pressure, RBC deformability was not altered by any of the three calcium antagonists. We conclude that a two-week treatment with calcium antagonist, at the effective vasodepressor dose, has no effect on RBC deformability in mild-to-moderate hypertensive patients.