Background Randomized controlled psychosocial treatment studies on selective mutism (SM) are lacking. Method Overall, 24 children with SM, aged 3–9 years, were randomized to 3 months treatment (n = 12) or wait list (n = 12). Primary outcome measure was the School Speech Questionnaire. Results A significant time by group interaction was found (p = .029) with significantly increased speech in the treatment group (p = .004) and no change in wait list controls (p = .936). A time by age interaction favoured younger children (p = .029). Clinical trail registration: Norwegian Research CouncilNCT01002196. Conclusions The treatment significantly improved speech. Greater improvement in the younger age group highlights the importance of an early intervention.