Renal biopsy specimens of 16 adult patients with nephrotic syndrome and focal glomerular sclerosis were examined by light and electron microscopy. Particular attention was paid to alterations of podocytes. Except for loss of foot processes, five patients had no podocyte changes, five had mild changes and six had severe changes. Of the last group (group III), four patients were heroin addicts, the fifth had infectious mononucleosis and the sixth, an apparent idiopathic disease; five patients were males, 16 to 25 yr old. Podocyte changes consisted of cytoplasmic degeneration, detachment of epithelial cells from basement membranes, with filling of resulting space by cell debris and new membranes. Underlying capillaries were often collapsed. Repeat biopsies in three patients in group III revealed progression of lesions, paralleling rapid clinical deterioration. It is concluded that some cases of focal glomerular sclerosis are associated with severe damage to podocytes which may be caused by drugs, infection or unknown factors and may contribute to the development and progression of the glomerular lesions.