The terminology of megaureters, their histological findings and etiology vary greatly in the literature. Primary obstructed megaureters must be clearly differentiated from megaureters caused by infravesical obstruction. The characterization and therapeutical consequences of the two types also differ. In a retrospective study, 43 adult patients with 54 primary obstructed megaureters (male:female = 30:13) were studied. This diagnosis excluded infravesical obstruction, reflux and neurogenic disorders. The main symptoms were flank pain, infection and hematuria. An operation was performed in 25 patients; 29 were treated conservatively. The criteria for surgery are discussed in relation to symptoms, functional reserves of the kidney, transportation capacity of the upper urinary tract, and histological changes of the narrow segment of the ureter. Operative treatment should be limited to well-defined complications, such as deterioration of kidney function or recurrent infection, to avoid its discredit as merely a cosmetic procedure.