We report a 5-year-old girl with adenosine deaminase (ADA) deficiency who was asymptomatic during the first years of life. At 3 years of age, she developed chronic and recurrent sinopulmonary infections, and at 4 1/2 years of age she had one major infection with Streptococcus pneumoniae (bacteremia and septic arthritis of the hip). Immunologic evaluation at 5 years of age revealed persistent lymphopenia, decreased helper-suppressor T cell ratios, and low proliferative responses to mitogens. The IgG, IgM, and IgA levels were normal; the IgG2 level was low normal or below normal. The patient had specific antibodies against toxoids and viral antigens but failed to produce antibodies against Haemophilus influenzae type b and pneumococcal polysaccharides. Although no symptoms of allergy were present, she had persistent eosinophilia and elevated IgE levels. The patient had 0.6% of normal ADA activity in erythrocytes and approximately 1% of normal ADA activity in peripheral blood mononuclear cells. Beginning at 6 years of age, she was treated with weekly injections of polyethylene glycol-modified bovine ADA. This treatment was well tolerated and effectively reversed the biochemical consequence of ADA deficiency. Concomitantly, she improved clinically and her T lymphocyte numbers and blastogenic responses to mitogens in vitro became normal. The late onset of clinical symptoms and relatively benign clinical course in this patient emphasize the need to consider ADA deficiency in a broad spectrum of immunodeficient children.