Primary monosymptomatic nocturnal enuresis (PMNE) is the most frequent (85%) type of enuresis in children. It remains a diagnostic and therapeutic challenge to establish its etiology and implement a proper treatment.The aim of the study was to establish the causes of PMNE in children on the basis of own investigations and assess factors having influence over PMNE etiology, which would enable the choice of effective therapy.The study concerned 85 children with PMNE aged from 5 to 15 years. The patients were under the care of Nephrology Outpatient Clinic at Institute of Mather and Child in years 2009-2014. 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. Statistical analysis was performed using R software.In all patients, we have successfully detected the etiology of children of PMNE. The basic, equally frequent (62.3%), PMNE etiopathogenetic factors turned to be: too small bladder capacity resulting from the detrusor hyperactivity, and night polyuria mainly caused by vasopressin deficiency or abnormal eating and hygienic habits, occurring separately or in conjunction with each other. Too small bladder capacity occurred mainly (37.6%, group C) as the only etiological factor of PMNE, and in 24.7% (group A) in a conjunction with nocturnal polyuria due to decreased excretion of vasopressin. Night polyuria was caused by the deficiency of vasopressin in most cases (37.6%) and occurred mainly (24.7%, group D) in a conjunction with small bladder capacity, and rarely alone (12.9%, group B). In 24.7% (group A) it appeared due to eating and hygienic abnormal habits. We have proved statistically significant differences in mean voiding frequency and volume (p<0.0001) between groups A-B and C-D. Mean morning urine specific gravity (p<0.0001) also differed significantly between group C and B (p<0.0001) as well as C and D (p=0.0004).PMNE in all patients was attributed to specific causes outside the circle of psychological disorders what reduced patient stigmatization. PMNE etiology is very complex and diverse. It still remains a challenge and requires and individual diagnostic and therapeutic approach. Voiding frequency above 8 daily with voiding volumes usually below 100 ml suggest etiology connected with small bladder capacity, while morning urine specific gravities below 1.021 g/ml can be connected with vasopressin deficiency or excessive fluid intake before the bedtime. The developed diagnostic approach along with borderline values are hints that can aid physicians in establishing PMNE causes.