To evaluate the benefits and harms of different agents (used singularly or in combination) compared with placebo, no treatment or any other agent for: (1) the prevention of severe kidney disease in patients with HSP without kidney disease at presentation; (2) the prevention of severe kidney disease in patients with HSP and minor kidney disease (microscopic haematuria, mild proteinuria) at presentation; (3) the treatment of established severe kidney disease (macroscopic haematuria, proteinuria, nephritic syndrome, nephrotic syndrome with or without acute kidney failure) in HSP; and (4) the prevention of recurrent episodes of HSP-associated kidney disease.