Thirty-three children with post-streptococcal acute glomerulonephritis, age mean: 8.3 years (range: 6 - 12) were studied prospectively. Mean initial hematocrit (Hct) was 31.6% with 90% showing Hct under the normal lower limit for this age group. Reticulocyte index (RI) was < 0.5 in half of the cases. Serum iron concentration, total iron binding capacity (TIBC) and percentage of transferrin saturation were normal for this age group although 75% of the children had increased serum ferritin levels. At the time of discharge, Hct increased to 35.1% but 44% still had anemia. Hct increased spontaneously for 105 days stabilizing at 38%. Based on Hct changes, 3 groups were defined: Group I (3 individuals): normal upon discharge; Group II (19): partial recovery at discharge, slow recovery stabilizing after 105 days; Group III (11): lower Hct, slower recovery but with RI significantly higher than group II (0.96 vs 0.45 p < 0.01). Our data suggest that although hemodilution is present in all, it may be considered the solely factor only in 3 cases (Group I). In group II, evidence of bone marrow depression was indicated by the low RI. On the other hand, the intense anemia that could not be justified only by hemodilution and marrow depression in group III, suggests other pathogenic factors.