Nocturia is one of the most bothersome of all urologic symptoms, yet even a rudimentary classification does not exist. We herein propose a classification system of nocturia based on a retrospective study. The records of 200 consecutive patients with nocturia were reviewed. Evaluation included history, micturition diary (including day, night, and 24-hr voided volume), postvoid residual urine (PVR), and videourodynamic study (VUDS). Functional bladder capacity (FBC) was determined to be the largest voided volume in a 24-hr period. The etiology of nocturia was thus classified into one of three groups: nocturnal polyuria ([NP] in which voided urine volume during the hours of sleep exceeds 35% of the 24-hr output), nocturnal detrusor overactivity ([NDO] defined as nocturia attributable to diminished bladder capacity during the hours of sleep), and mixed (NP+NDO); polyuria (24-hr urine output >2,500 cc) was classified separately. There were 129 women and 65 men ranging in age from 17 to 94 years (x=59). Overall 13 (7%) had NP, 111 (57%) NDO, and 70 (36%) had a mixed etiology of their nocturia (both NP and NDO). Forty-five (23%) also had polyuria. These data confirm that the etiology of nocturia is multifactorial and in many instances unrelated to the underlying urologic condition. Nocturnal overproduction of urine is a significant component of nocturia in 43% of patients, most of whom will also have NDO. We believe that treatment should be directed at both conditions.