We evaluated the clinical significance of measurement of urinary neopterin levels in primary immunodeficiencies and persistent infections exclusively at afebrile or asymptomatic periods. Despite the examinations at afebrile or asymptomatic periods, urinary neopterin levels were elevated in some patients with primary immunodeficiencies and in patients with persistent infections of human immunodeficiency virus (HIV) or Epstein-Barr virus (EBV). Therefore, urinary neopterin measurement at afebrile periods will be useful as one of the screening tests in the detection of such disorders among children with occasional episodes of infections.