We encountered 4-year-old boy who developed paroxysmal cold hemoglobinuria (PCH) in the middle of August. He was admitted due to progressive jaundice and pallor following fever and nausea. Laboratory data revealed severe anemia, increased serum indirect bilirubin and LDH, and decreased serum haptoglobin. Direct/indirect Coombs tests were positive. These findings yielded a diagnosis of autoimmune hemolytic anemia. Serological test for syphilis was negative. The patient's symptoms and signs promptly subsided after treatment with prednisolone, which was started on the 2nd hospital day. The patient has been in a disease-free state for 16 months without any medication. After discharge, he was finally given a diagnosis of PCH because a Donath-Landsteiner test was positive. The antibody belonged to an IgM subclass and showed anti-I specificity. Considering that development of PCH is common in winter, this case was unique because it developed in August. We speculated that exposure to a cool air-conditioned atmosphere prior to hospitalization and the cooling mechanism of the body after admission were involved in the summer onset of PCH and its prolonged clinical course.