Patients with the idiopathic nephrotic syndrome (INS) with steroid treatment failure, who did or did not have a history suggesting food intolerance, were systematically evaluated for food allergy. We evaluated 42 cases for sensitization to usual foods. The following results were obtained: skin tests (intradermal, Stallergenes Lab) were positive in 16 out of 42 cases; total serum IgE was elevated in 12 of 42 patients; specific IgE (RAST) was positive in eight of 42 patients; the basophil histamine release test (HRT) was positive in 20 of 42 patients. For each of these tests, the results in the INS patients with steroid treatment failure were significantly different from controls. The following exclusion or challenge tests were carried out. In seven cases, an elemental diet was used; proteinuria decreased in one case, but acceptance of the diet was poor in the other cases. In 27 cases, a limited diet with exclusion of one or several foods was used. A complete remission was induced by this diet in seven steroid-dependent INS cases, allowing steroids to be discontinued during a follow-up of one to five years. However, relapses were frequent. More recently, an oligoantigenic diet was prescribed for 10 days without modifying steroid therapy. In 13 INS patients (5 steroid-dependent, 3 steroid-resistant, and 5 multiple relapses), 24-hour urine protein was significantly reduced (P less than 0.05). Proteinuria decreased by more than half in nine patients, with complete disappearance in five cases.(ABSTRACT TRUNCATED AT 250 WORDS)