Abstract The urinary excretion of lipoproteins and the possibility of catabolic alterations on glomerular filtration were investigated in four nephrotic subjects differing in etiology, serum lipoprotein profile, and 24 hr urinary output of protein and lipids. The apolipoproteins and lipoproteins of urine were compared with those of serum with respect to distribution profile, physical properties, and composition. Lipoprotein particles resembling the serum very low, intermediate, low, and high density lipoproteins (VLDL, IDL, LDL, and HDL, respectively) in density, particle size, and morphology were isolated from the urine. As expected from molecular sieving effects during glomerular filtration, the urinary HDL were more abundant than the lower density lipoproteins even when the plasma LDL was elevated markedly. However, little sieving effect was seen within the urinary HDL, which comprised a broad spectrum of particle sizes including the larger HDL 2, whose average diameter was similar to that of the plasma HDL. A sieving effect was not seen in the urinary LDL, except for a greatly increased proportion, about 20% of total particles, of HDL-like species. Intact apolipoproteins were not found in the concentrated urinary fraction isolated by ultrafiltration between the limits of 10 4 and 5 × 10 4 daltons. On the basis of immunoreactivity, gel electrophoresis, and amino acid composition, apolipoproteins B and AI are the major and minor proteins, respectively, of urinary LDL, and apo B is the major protein of the urinary IDL and VLDL. Apolipoproteins AI, AII, CI, CIII, and possibly AIV were isolated from the urinary HDL. As much as 20% of the protein moiety of the urinary HDL appeared to be large apolipoprotein fragments with molecular weights and isoelectric points similar to those of apo CII and apo CIII. The fragments were derived in part from apo AI, the least acidic form of which was lost preferentially. The lower density classes of urinary lipoproteins also appeared to have lost apo E and apo C's and to have undergone partial proteolysis. Apparently, the surface-exposed, readily exchangeable apolipoproteins are subject to proteolytic degradation upon glomerular filtration.