In 1968 an outbreak of 348 cases of acute poststreptococcal glomerulonephritis (AGN) was observed in Maracaibo, Venezuela. During the year, the epidemic had three peaks of incidence. Districts with better sanitation showed a lower incidence of disease than those with less adequate facilities. Endemic cases occur every year. Lowering of serum complement activity was observed in 96% and hypertension in 90% of the patients. The mortality rate in the acute phase during the epidemic was 1.31%. A history of antecedent infection was found in 39% of the cases. 70% of these infections were in the upper respiratory tract. During 1973-74, 120 of the patients (19 adults and 101 children) were reexamined. All but one had been completely asymptomatic. Sixteen patients (13.3%) had one or more of the following abnormalities (group A): low CCr, microscopic hematuria, proteinuria of 1 g/day or more and hypertension. One hundred and four patients (86.7%) were normal by all clinical and laboratory parameters tested (group B). The incidence of persisting disease, as judged by biochemical findings, was significantly higher (P less than .01) in adults (36.7%) than in children (8.9%). Percutaneous renal biopsies from 7 patients of group A, and 8 patients from group B, were studied by light microscopy and immune histology. All biopsies from group A patients had evidence of advanced glomerular disease by light microscopy and by immune histology. Seven biopsies from group B patients were also abnormal showing mild changes with corresponding immunologic findings. Only in one patient was the biopsy completely normal. Our studies indicate that significant numbers of patients progress towards chronicity after epidemic, poststreptococcal AGN. Such progression is more common in adults than in children.