Henoch-Schönlein purpura is the most common vasculitis of childhood. This study investigated the values of hematologic indices that can help predict internal organ involvement. The study included 112 patients followed up between January 2007 and May 2017 and 81 healthy children. Leukocyte, neutrophil, monocyte, lymphocyte and platelet counts, neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and C-reactive protein (CRP) levels were compared between patients with and without internal organ involvement. Overall, 57 (50.8%) patients had internal organ involvement. Leukocyte, neutrophil, and monocyte counts, NLR, and CRP levels were significantly higher in patients with internal organ involvement than in patients without internal organ involvement. There was no difference between the groups in terms of lymphocyte count, platelet count, and PLR. The cutoff values were found to be ≥10.8×10/L [area under the curve (AUC), 0.734] for leukocyte, ≥6.0×10/L (AUC, 0.665) for neutrophil, ≥0.710×10/L (AUC, 0.681) for monocyte, ≥3.95×10/L (AUC, 0.609) for NLR, and 2.41 mg/dL (AUC, 0.635) for CRP. Logistic regression analysis revealed that leukocyte count is a risk factor for internal organ involvement. Leukocyte, neutrophil, monocyte counts, NLR, and CRP levels are useful in predicting internal organ involvement in the acute phase of Henoch-Schönlein purpura. Leukocyte count is an important risk factor for internal organ involvement and its predictive value is more reliable than the other hematologic indices.