Mental retardation in combination with proteinuria and a slight hyperlipoproteinemia was found in three brothers. The increased urinary protein excretion was dominated by albumin and the low molecular weight proteins retinol-binding protein (RBP) and beta2-microglobulin, indicating the presence of proximal tubular dysfunction. However, there was no glucosuria, phosphaturia or amino aciduria and the renal concentrating and acidification capacities were normal. A kidney biopsy in one of the patients revealed morphologic evidence of glomerular damage but a normal tubular structure. A mild hyper-beta-lipoproteinemia was found in the patients but not in their healthy siblings. The cause of this syndrome, hitherto not described, is unknown.