ABSTRACT Purpose: We investigate the nature of enuresis episodes in monosymptomatic nocturnal enuresis using a fluid provocation model. Materials and Methods: The study included 18 children 7 to 13 years old with monosymptomatic nocturnal enuresis. Based on basal home recordings patients were subgrouped into those with a normal nocturnal urine output and those with nocturnal polyuria (mean nocturnal urine production on wet nights exceeding 130% of functional bladder capacity, normal functional bladder capacity for age provided). Children were admitted to the hospital for 4 consecutive nights. After an adaptation night all children received orally 25 ml/kg water, 30 minutes before bedtime on the remaining 3 nights. A cordless alarm device enabled registration of enuretic episodes from another room and diapers allowed the measurement of enuresis volumes. Post-void residual volumes were measured by ultrasound. Pelvic floor electromyography was continuously recorded throughout the night, and its association to bladder emptying was investigated. Results: A total of 95 enuresis and 14 nocturia episodes were recorded. Significantly more enuresis episodes were registered on nights with oral fluid load, whereas no increase in number of nocturia episodes was seen. Of the enuresis episodes 46 were associated with incomplete bladder emptying (post-void residual volume greater than 10% of total bladder volume at time of enuresis). No difference between patient groups regarding post-void residual volume was seen. Abnormal bursts of electromyography activity were associated with incomplete micturitions. Conclusions: Enuresis nocturna episodes in polyuric and nonpolyuric patients are frequently incomplete micturitions. The present findings question the definition of nocturnal enuresis episode as normal complete voiding.