Twelve patients with nephrotic syndrome in the first year of life (NSFY) are described. All the patients showed edema, hypoalbuminemia (0.6-1.9 gr/dL), proteinuria (> 40 mg/m2/hr), nephromegaly at excretory urography and low birthweight. Prematurity and hepatomegaly were present in half of them. 8 had hypercholesterolemia (240-720 mg/dL). Creatinine was 0.3 to 1.5 mg/dL, and urea 12.8-57 mg/dL. Uric acid was elevated in 3 patients (>7 mg/dL). IgG levels were diminished in 2 and IgM elevated in 3 cases. TORCH screening revealed one case for hepatitis-A and cytomegalovirus (patient and mother), and one patient with herpes (mother negative). Eight patients had diarrhea and dehydratation, and 7 of them had sepsis (5 deaths) caused by Klebsiella, S. aureus and Salmonella sp. in 3 different cases. Five patients developed chronic renal failure. Only one patient is alive with renal transplantation. One patient showed spontaneous remission and two had remission with treatment(prednisone and prednisone + cyclophosphamide). Renal biopsy in 7 patients showed minimal changes and intersticial fibrosis, 2 patients had focal and segmentary glomerulosclerosis and one patient had mesangial diffuse sclerosis. All cases had tubular lesions but not characteristic of classic microcystic disease. Immunofluorescence was negative and EM showed podocyte fusion and epithelial cells villous transformation. Only one case had vascular obliteration. This study shows the classic evolution of NSFY with intercurrent infections and death secondary to sepsis. Severity of the lesions did not correlate with histological changes. There was no familial incidence nor similar infectious etiology. Interstitial fibrosis was the main factor associated with chronic renal failure. Although clinical evolution and follow up are different from childhood nephrotic syndrome, the similarities in histological characteristics suggest a common etiology, with a more aggressive evolution in the immature kidney of the NSFY.