In 27 perinatally human immunodeficiency virus type 1 (HIV-1)-infected children, we measured, by immunonephelometry, the kappa/lambda light chain ratio (KLR) of serum immunoglobulins. The latter is a recently available laboratory index reflecting the balance between the synthesis of K isotypes and L isotypes. KLR was consistent over time in each subject, and was significantly lower than that of an age-matched normal population, independently of disease status and therapy. These data indicate a bias, in these subjects, to produce preferentially lambda rather than kappa light chains, contributing to the multiple B-cell abnormalities in HIV-1-infected children.