OBJECTIVE Detection of possible effects on renal function parameters of long-term antihypertensive treatment administered to patients with diabetes mellitus. METHODS Prolongation of an open trial, initially planned for one year, on the effect of felodipine 10 mg/day in diabetics (Fortschr. Med. 109 (1991), 53-56). Continued treatment of 7 hypertensive insulin-dependent diabetics (mean age 58 +/- 3 years, HbA1 8.2 +/- 0.3%) over a period of between 24 and 32 months. RESULTS The lowered blood pressure (from 160/94 to 150/82 mmHg; p less than 0.05) persisted. In the first 6 months of treatment, urinary albumin excretion (UAE) remained constant. Thereafter it increased (p less than 0.05 as compared with the figure seen after 6 months' treatment) to a degree that varied from one patient to another; overall, the UAE figures showed a relatively large intra-individual fluctuation during long-term therapy. Metabolic parameters remained uninfluenced by felodipine. CONCLUSIONS It cannot be excluded that progression of diabetic renal disease cannot be adequately prevented by long-term treatment with calcium antagonists such as felodipine.