Alagille syndrome is characterized by the association of chronic cholestasis with a paucity of interlobular bile ducts and a distinctive facies together with cardiovascular, skeletal and eye abnormalities. We examined the kidneys of 26 patients with this syndrome; 22 were under 3 years of age and 4 were 4, 6, 12 and 17 years old, respectively. Eighteen showed glomerular lesions of variable severity characterized by a mesangiolipidosis. In the 8 lesser affected patients light microscopy (LM) disclosed a fibrillar appearance of the mesangium, and electron microscopy (EM) showed lipid vacuoles widespread in the mesangial matrix. In the 10 patients who were affected to a greater degree LM and EM showed, in addition to the mesangial matrix changes, the presence of mesangial foam cells. Clinical signs of renal involvement were mild in all patients except for one who died from chronic renal failure at 8 months of age. The extent of mesangiolipidosis was not related to age but to the degree of cholestasis, the most severe lesions being observed in patients aged 3, 6, 8, and 14 months. The glomerular lesions observed in Alagille syndrome are strikingly similar to those observed in adults with lecithin-cholesterol acyl transferase deficiency and other conditions characterized by an increase in plasma lipoproteins rich in free cholesterol and in phospholipids. We conclude that glomerular involvement should be added to the characteristic features of Alagille syndrome. Also we found that the lipid deposition in the glomeruli of patients with Alagille syndrome is related to an abnormal lipid metabolism, which is the consequence of severe cholestasis. The most striking feature of our study is the early detection of the glomerular lesions, contrasting with the lack of overt clinical renal disease. Renal failure may be a major complication for patients with this syndrome in adulthood.