A 2-month-old boy was admitted to our hospital because of a 4-day history of fever, cough, purulent nasal discharge, and skin lesions. The patient had been born via spontaneous vaginal delivery at 39 weeks' gestation, and he was apparently normal at birth, excluding low weight (2500 kg). However, his mother had not received any prenatal care; therefore, a maternal laboratory evaluation was carried out just before delivery. Her serologic results for hepatitis B and hepatitis C, toxoplasma, and the rapid plasma reagin (RPR) test for syphilis were negative, but serology for human immunodeficiency virus (HIV) was positive. This maternal HIV infection was not previously known. Diagnosis of HIV in the neonate was then confirmed by DNA detection (polymerase chain reaction), and antiretroviral treatment was started.