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The object of the study was to test whether high dose ascorbic acid (AA) could normalize glomerular hyperfiltration in insulin-dependent diabetes mellitus (IDDM) patients. A prospective, double-blind, randomized, placebo (tartaric acid, TA) controlled study design was used, with parallel treatment lasting 4 weeks. Measurements were made before and after treatment, on 24 normoalbuminuric, normotensive male IDDM patients, who were randomized to ascorbic acid (n = 12, age 35 years (18-39), diabetes duration 12 years (2-12), BP 128/82 mmHg (SD 14/6)), or to placebo (TA) (n = 12, age 30 years (19-36), diabetes duration 8 years (2-17), BP 119/75 mmHg (SD 9/7). The intervention consisted of 6 enterosoluble tablets of 500 mg AA or 213 mg TA, twice a day, being daily doses of 6 g AA or 2.55 g TA. No significant differences in any of the parameters measured were seen, when comparing results following AA or placebo treatment. The glomerular filtration rate (GFR, clearance of 125I-iothalamate) was unchanged while effective renal plasma flow (ERPF, clearance of 131I-hippuran) tended to decline in both groups. The GFRs before and after treatment in the AA-treated group were 141 (SD 15) and 134 (SD 12) ml min-1 1.73 m-2; NS (2p = 0.09). In the TA-treated group they were 142 (SD 19) and 137 (SD 16) ml min-1 1.73 m-2; NS (2p = 0.20). The ERPFs in the AA group were 584 (SD 93) and 545 (SD 47) ml min-1 1.73 m-2; (2p = 0.06). In the TA group they were 618 (SD 108) and 574 (SD 98) ml min-1 1.73 m-2 (2p = 0.03). The filtration fractions (FFs) in the AA group were 0.244 and 0.246 NS.(ABSTRACT TRUNCATED AT 250 WORDS)