A prospective study was performed to verify whether early administration of prednisone could be useful in preventing the development of nephropathy in anaphylactoid purpura. Only patients without signs of nephropathy upon initial presentation entered into the study. A total of 84 patients received delta-prednisone (1 mg/kg per day per os for 2 weeks), and 84 patients did not receive steroids. The patients were followed for 24-36 months. None of the 84 patients treated with steroids and 10 (11.9%) of the 84 control patients developed nephropathy 2-6 weeks after the acute episode. In 2 other patients of the untreated group, signs of renal involvement appeared 2 and 6 years after the acute episode respectively. The difference in the prevalence of nephropathy between the two groups is highly significant (P less than 0.001).