Using objective methods of evaluation, endopyelotomy was successful in relieving obstruction in 87% of cases. If inaccuracies in the interpretation of DRG, i.e. a large pelvic capacity and poor renal function, are accounted for, the results of WT as early as 24 h after removal of the splint correlated with DRG. Drainage patterns on DRG did not deteriorate during the first year. An early post-operative evaluation with WT or DRG, as appropriate, is thus sufficient evidence of the success of the procedure. Evaluations repeated during the first year after endopyelotomy may be unnecessary.