By means of immunonephelometry, determinations of a number of high molecular weight (HMW) and low molecular weight (LMW) proteins in urine and serum were undertaken in 42 consecutive patients, who had been subjected to jejuno-ileal bypass surgery for treatment of massive obesity two to six years before the study. Five patients demonstrated a distinct LWM proteinuira, ie excessive excretion of free-light lamdba and kappa chains of immunoglobulin and beta-2-microglobulin. The creatinine clearance was normal in four of these five patients. This LMW-proteinuria group differed from the remaining 37 patients in several respects. First, they had lost weight more effectively (P less than 0.01); secondly, they exhibited secondary hyperparathyroidism (P less than 0.05), increased levels of alkaline phosphatase and low serum concentrations of bicarbonate (P less than 0.001). It is suggested that LMW proteinuria may be a manifestation of secondary hyperparathyroidism.