This study demonstrates a comparative assessment of the causes of nocturia in young and elderly patients with overactive bladder (OAB). We used the database of 245 patients over 18 years (117 men, 128 women) with OAB (more than 8 micturition per day), with presence or absence of urgent urinary incontinence and nocturia (an average of 2.5 episodes per night). All the patients had to complete a diary within 3 days indicating the time of the urge to urinate, urine volume per micturition and sleep time (when the patient is actually asleep before waking time). These patients were divided by sex and also into three age groups: younger than 60 years, 61-69 years and over 70 years. A diary was used to determine the nocturia index, nocturnal polyuria index, index of nocturnal bladder capacity. The most obvious reason for nocturia in patients with OAB was the polyuria and reduced nocturnal bladder capacity, which are not dependent on age. In young patients (< or = 60 years) nocturia was the result of the decrease of nocturnal bladder capacity and was evaluated by IBC. For the patients older than 70 years, the most common cause of nocturia was nocturnal polyuria, which was estimated by the index of nocturnal polyuria.