We report the case of a 3-year-old boy with a history of repeated neutropenia and infection along with a significant family medical history. The diagnosis was cyclic neutropenia based on genetic testing. He had experienced frequent repeated episodes of fever since age 8 months, and he was found to have neutropenia when hospitalized due to acute otitis media at age 13 months. We obtained serial data of complete blood cell counts for 6 weeks when he was 3 years and 4 months old, which confirmed a cyclic increase and decrease in the number of neutrophils. ELANE gene testing was performed after obtaining written informed consent from the parents. A missense mutation was identified. At the parents’ request, we ran further testing and identified the same mutation in the father and paternal grandmother. Thus, 3 generations of kindred had the ELANE gene mutation. Because of repeated infections, prophylactic sulfamethoxazole trimethoprim was started from age 3 years and 10 months, resulting in a considerable decrease in the frequency of infections. Since age 4 years and 11 months, cyclic neutrophil over about 3 weeks has persisted, but there have been no serious infections requiring hospitalization.