Fifty-six type I diabetic patients with microalbuminuria (albumin excretion rate 20-200 micrograms/min) were characterized as to sex, age, duration of diabetes, smoking habits, blood pressure, glomerular filtration rate, urinary NC1 (the carboxy-terminal domain of collagen IV), and Tamm-Horsfall protein excretion rate. Albumin excretion rate was considered a sign of glomerular damage, NC1 excretion rate a measure of renal basement membrane turnover, and Tamm-Horsfall protein excretion rate a marker for distal tubular function. There were no differences between males and females and between smokers and nonsmokers with respect to blood pressure, body-mass index, albumin excretion rate, glomerular filtration rate, excretion rate of NC1, and Tamm-Horsfall protein. As a group, the patients with microalbuminuria had normal glomerular filtration rate, excretion rate of NC1, and Tamm-Horsfall protein. The latter was influenced by glycosylated hemoglobin (HbA1c) levels, especially so in patients with an albumin excretion rate less than the median value of 53.0 micrograms/min (r = -0.61, p < 0.01). Furthermore, both excretion rate of NC1 and Tamm-Horsfall protein were increased in patients with high glomerular filtration rate > or = 130 mL min-1 1.73 m-2). There was no association between glomerular filtration rate and HbA1c levels. As glomerular filtration rate is related to kidney size, these observations suggest that patients with a high glomerular filtration rate have an increased mass and turnover of tubular basement membrane.(ABSTRACT TRUNCATED AT 250 WORDS)