Hepatitis B surface antigen was detected by radioimmunoassay in the sera of 18 out of 50 (36%) children with nephrotic syndrome and in 28 of 61 (45.9%) controls. Immunofluorescent studies of kidney biopsies showed HBsAg, IgG, IgM and C3 deposits in a granular pattern in the biopsies of 12 children with nephrotic syndrome and in none of the control kidney biopsies, even though there was no significant difference between the frequencies of HBsAg in the sera of these two groups. We conclude that these findings are indicative of an aetiologic role for HBsAg in these patients.