Introduction Nocturnal enuresis has been found a common symptom among children with breathing problems and sleep apnea. The purpose of this study was to evaluate the therapeutic and placebo effects of slow maxillary expansion on nocturnal enuresis. Material and methods Four children with enuresis aged 7–12 years were selected. Rigid acrylic expansion appliances were fabricated and delivered to them. Frequency of enuresis was recorded by the parents during three stages: 1) before appliance delivery; 2) after appliance insertion without expansion; and 3) during expansion and retention. Results The frequency of wetting decreased significantly during the period of appliance use without expansion. During the expansion and retention phase, two patients became completely dry, and two patients improved significantly. Conclusions Maxillary expansion can have a positive effect on the treatment of nocturnal enuresis. Also, the placebo effect of the expansion appliance has significant effects on enuresis.