We prospectively investigated the evolution of proteinuria in 52 type I diabetics over 7.8 ± 0.3 (mean ± SE) yr and in 61 type II diabetics over 6.4 ± 0.3 yr. Measurements of renal protein clearance were performed serially, and the time course of proteinuria was classified in each subject based on a threshold albumin clearance of 11 nl/s, equivalent to a urinary albumin excretion rate of 30 μg/min. The classification based on this threshold yielded four distinct patterns of albuminuria: minimal, intermittent, progressing, and established. These patterns occurred in both type I and type II diabetics independently of the duration of follow-up. This study has identified a pattern of intermittent microalbuminuria that is also associated with transient elevations of transferrin and IgG clearances. The relationship of clinical and biochemical parameters to proteinuria patterns was evaluated. No relationship was detected between proteinuria patterns and glycemic control in either type I or type II diabetics. In type I but not type II diabetics, established proteinuria was associated with higher systolic blood pressure and decreased creatinine clearance. The phase of intermittent proteinuria detected in this study may represent a reversible stage in the development of diabetic nephropathy, but the factors that trigger the transition to progressing proteinuria remain obscure.