We investigated the production of cytokines by peripheral blood mononuclear cells (PBMC) and serum cytokine concentrations in children with steroid-sensitive idiopathic nephrotic syndrome (SSNS). PBMC from patients off treatment were collected during remission and relapse and cultured in medium alone or stimulated with calcium ionophore plus phorbol myristate acetate. Control PBMC were taken from healthy age-matched children. IL-2 was measured by bioassay, IL-4 by immunoradiometric assay, and IL-8 and IFN-γ by ELISA. After 24 h culture without stimulation, IL-2, IL-4 and IFN-γ were not detectable in the supernatant in any of the children. After stimulation, the supernatant concentrations of IL-2 (median 172 U/ml at 24 h) and IL-4 (160pg/ml at 24 h; 210pg/ml at 72 h) were significantly increased in relapse compared with remission (IL-2 37 U/ml; IL-4 65pg/ml and 60pg/ml) and controls (IL-2 69 U/ml; IL-4 40pg/ml and 40pg/ml) (P <0.05). The concentration of IFN-γ was not significantly increased in relapse compared with remission and controls (600, 325, and 145 U/ml, respectively, at 72 h). IL-8 concentrations were similar in relapse, remission and controls with stimulation (median 32, 40 and 40 ng/ml, respectively) and without (30, 17 and 10 ng/ml). IL-2 was not detectable in serum, but IL-4, IL-8 and IFN-γ were measurable in about half the patients, both in relapse and remission, though were virtually undetectable in controls. We conclude that relapse of SSNS in children is associated with T lymphocyte activation with release of IL-2, IL-4 and IFN-γ.