Background: Patients with Hb Mizuho may be splenectomized at a young age to decrease their need for blood transfusions. Observations: Transfusion-dependency decreased dramatically in a 4-year-old white boy with Hb Mizuho after splenectomy. Surprisingly, he developed reticulocytosis (>1000×10⁹/L) with a peak reticulocyte percentage of 49%, and erythrocyte abnormalities, including Heinz bodies, Howell-Jolly bodies, and basophilic stippling. Manual reticulocyte counting and flow cytometric measurement with anti-CD71 antibodies supported a truly elevated reticulocyte count. Conclusions: We propose possible explanations for the extreme reticulocytosis that arose postsplenectomy and compare the reticulocyte count in the present case with previously published cases.