The hyperlipidemic Imai rat was originally developed as an animal model of spontaneous hyperlipidemia. We report the natural course of the Imai rat up to 32 weeks of age focusing on renal pathology. The degree of proteinuria, which first appeared at 8 weeks, increased with age, and all Imai rats developed heavy proteinuria (mean, 228 mg/24 h) with impaired renal function (mean BUN, 78.7 mg/dl) at 32 weeks. Histologic changes of the glomeruli were characterized by focal and segmental sclerosis and hyalinosis. Both the percentage of affected glomeruli and the severity of each affected glomerulus were progressively increased with age. The immunofluorescence and electron-microscopic findings were also comparable to those of focal glomerulosclerosis (FGS) in humans. The serum levels of total cholesterol, triglyceride, and phospholipid in Imai rats were significantly higher than those in normal Sprague-Dawley rats at 8 weeks of age, and progressively increased thereafter. The proteinuria, glomerular involvements, and hyperlipidemia were generally less severe in the females than in the males. We conclude that the hyperlipidemic Imai rat, a naturally occurring animal model of FGS, is useful in studying the pathogenesis of FGS and the renal effects of hyperlipidemia in humans.