Primary monosymptomatic nocturnal enuresis is the most frequent (85%) type of enuresis in children. Establishing its causes enables the choice of effective therapy.To establish the causes of primary, monosymptomatic nocturnal enuresis in children on the basis of own investigations.The study concerned 47 children (36 boys and 21 girls) with primary, monosymptomatic nocturnal enuresis aged from 5 years 5 months to 15.5 years. The patients were under the care of Nephrological Outpatient Clinic at the Institute of Mother and Child in the years 2009-2013. The detailed medical history, physical examination as well as laboratory investigations of blood and urine and radiological investigations of the urinary tract, were carried out.The most frequent causes of nocturnal enuresis in the studied groups was destructor over-activity (55,3%). Other causes were: nocturnal polyuria (21,3%) and destructor overactivity together with polyuria (12,8%). In a lower percentile (10,6%) the causes of nocturnal enuresis were incorrect habits concerning drinking, time and amount of fluids taken before sleep and the lack of habits to pass urine before sleep.1. Primary monosymptomatic nocturnal enuresis is a significant underestimated problem. 2. Etiology of nocturnal enuresis in the developmental period has heterogenous origin and requires individual diagnostic and therapeutic approach. 3. Obtained results of investigations conclude that primary monosymtomatic nocturnal enuresis is conditioned by concrete organic and not mental causes as is quite often suggested, and in this way stigmatizing the population in which this disorder is found.