Early signs of renal dysfunction in glycogen storage disease type Ia (GSD Ia) are glomerular hyperfiltration and proteinuria. In a non-randomized study, the effect of captopril on the improvement of proteinuria in GSD Ia patients with microalbuminuria was investigated. A positive effect has been shown for the insulin-dependent diabetes mellitus patients. Microalbuminuria was defined as albumin/creatinine ratio (mg/mmol) more than 2.5 in spot urine. Nineteen (52.7%) out of 36 patients had microalbuminuria, and 8 patients received captopril at a dose of 1mg/kg per day. Microalbuminuria was evaluated periodically during the follow-up period. Of the captopril-treated patients, one was lost to follow-up. In the remaining 7 patients, urinary albumin excretion normalized in 3 patients (42.9%) and decreased at least by 50% in another 3 patients (42.8%) after 6 months of treatment. One patient, who was the oldest, did not have any benefit. In untreated patients, only two patients had a decrease in microalbuminuria of more than 50%. Patients with microalbuminuria had significantly higher blood lactate (p < 0.05) and plasma triglyceride (p < 0.01) concentrations and significantly lower blood bicarbonate concentration (p < 0.05) than those patients without it. Additionally, the patients with microalbuminuria had been diagnosed earlier than those without microalbuminuria (p < 0.05). Patients with microalbuminuria have more severe clinical and laboratory findings than those without microalbuminuria. Captopril at a dose of 1 mg/kg per day seems to be effective in at least 50% of GSD Ia patients with microalbuminuria.