Immunological studies were performed on 84 children with steroid-sensitive nephrotic syndrome as part of an investigation of the relationship between steroid-responsive nephrotic syndrome and allergy. Serum total IgE levels tended to be raised, particularly in children who had frequent relapses of nephrotic syndrome. Ten children had extremely high levels (greater than 1500 IU/ml) and several of them had neither a history of atopy nor any other identifiable cause. 25% of the children had at least one positive test for specific IgE antibody. IgE was not detected by immunofluorescence in renal biopsy tissue from 25 children, regardless of whether the child was in remission or relapse at the time of biopsy. Serum IgG and IgA levels were depressed particularly at the time of a relapse. Serum IgM tended to be raised and to remain so, even in children who had been in remission for more than a year. No clinically useful relationship was found between the frequency of HLA antigens and the occurrence or course of the syndrome, whether or not accompanied by atopy. Clinical and immunological features of atopy are more common in children with idiopathic nephrotic syndrome. This may be a causal or non-causal association. Pollen sensitivity is a rare cause of nephrotic syndrome; careful search for provocative agents may show other causes.