The effects of the calcium channel blocker, isradipine, on BP, urinary albumin excretion, plasma lipoproteins and natriuresis in albuminuric Type 1 (insulin-dependent) diabetic patients were assessed. Fifteen Type 1 diabetic patients aged 22-52 years were studied. All had elevated urinary albumin excretion (more than 30 mg/24h) based on several 24 h urine collections, and BP was normal (below 140/90 mmHg). After a placebo treatment period of eight weeks the patients were randomly assigned to two groups for a double-blind crossover study. Each patient received either 2.5 mg isradipine twice daily or placebo for eight weeks. Then, after 4 weeks (the wash-out period), each patient received the drug he or she had not taken before for another 8 weeks. Systolic blood pressure was lowered by 8 mmHg from 127 (114-139) mmHg (P less than 0.01) and diastolic by 5 mmHg from 81 (70-87) mmHg (P less than 0.03) during isradipine treatment. The 24 h urinary sodium excretion increased and no signs of volume expansion were observed during treatment with isradipine. Urinary albumin excretion and total body exchangeable sodium remained unchanged. During isradipine treatment the plasma concentrations of VLDL cholesterol and triglyceride decreased significantly (P less than 0.01) and the level of HDL cholesterol increased, but not significantly (P = 0.08). In conclusion, treatment of Type 1 diabetic patients, at risk of progressive clinical nephropathy, with the calcium channel blocker, isradipine, had beneficial effects on plasma lipoprotein levels and resulted in a reduction in BP. We did not find any effect of isradipine on urinary albumin excretion.