42 ileo-colonic orthotopic reservoirs (M.A.I.N.Z.) following radical cystectomy were evaluated with urodynamic studies. Long term results in 19 patients with a follow up longer than 36 months were compared to the findings of 23 patients with a shorter follow-up. Urodynamic evaluation showed a reservoir with large capacity and low pressure in both groups. Nocturnal incontinence occasionally occurred. Many factors may account for this problem. Instillation of hyperosmotic solution during cystometry resulted in increased amplitudes and frequency of intracavitary pressure waves: the nocturnal increase in urine osmolality could be in correlation with nocturnal incontinence.