Previous studies in males with lower urinary tract symptoms (LUTS) have shown that nocturia in some patients is associated with nocturnal polyuria whereas other patients have preserved circadian urine output rhythm. This outpatient study based on 7-dayss frequency/volume (FV) charts showed that patients with nocturnal polyuria and age-matched controls without nocturia had a diurnal variation in functional bladder capacity (FBC) with higher night-time values. In contrast, patients without nocturnal polyuria no diurnal variation was observed in FBC with lower day and night-time values than both controls and nocturnal polyuric patients. Nocturia volumes correlated significantly to daytime FBC in both patient groups. Voided volumes obtained from daytime pressure/flow urodynamic investigations correlated significantly with night-time FBC obtained from the FV chart. In patients with LUTS nocturia was caused by nocturnal polyuria in 10 of 23 patients and by diminished night-time functional bladder capacity in the remaining 13 patients. In conclusion, nocturia in males with LUTS referred for BPH evaluation is caused by a mismatch between nocturnal urine production and night-time FBC analogous with the pathophysiology of nocturnal enuresis. The evaluation of circadian urine production and FBC seems mandatory in the assessment of patients with nocturia and LUTS.