BACKGROUND: Children with borderline to mild intellectual disability (BMID) have been shown to be at increased risk for psychosocial problems. The presence of these psychosocial problems leads to parenting stress. Stepping Stones Triple P (SSTP) is a parenting support program to support parents with children with BMID and psychosocial problems. The aim of this study was to evaluate the effectiveness of SSTP compared to Care as Usual (CAU) in reducing psychosocial problems in children with BMID. METHOD: We conducted a randomized controlled trial in the Northern provinces of the Netherlands. Parents of children aged 5 to 12 with borderline (IQ 70 to 85) or mild (IQ 70 to 50) ID and psychosocial problems were invited. Psychosocial problems were identified using the Strengths and Difficulties Questionnaire (SDQ) parent report (>/=14). Measurements were assessed before the intervention (T0), immediately after the intervention (T1) and after a follow-up of six months (T2). SSTP takes 8 to 10 individual sessions of 40-90 minutes, provided over 10 to 12 weeks. CAU concerned any service, except SSTP. Primary outcomes were the child’s psychosocial problems (SDQ parent and teacher forms and the Eyberg Child Behavior Inventory, ECBI). Secondary outcomes were parenting stress (Parenting Stress Index, PSI) and parenting skills (Alabama Parenting Questionnaire, APQ). RESULTS: In total 209 parents of children aged 5 to 12 with BMID were allocated blindly to either SSTP (n =111) or CAU (n =98). In the intention to treat analyses, SSTP achieved no significantly better effect than CAU for the SDQ parent report, the ECBI and the APQ on the short- and long- term. In the short term, SSTP was significantly more effective than CAU for the SDQ teacher report (B = -2.25, 95% CI -3.79 to -0.71) and the PSI (B = -7.06, 95% CI -12.11 to -2.01). For both SDQ teacher report and PSI, there was no statistically significant effect in the long term. Dropout from SSTP was considerable (49%), with the effects being solely found in the adherent SSTP subgroup. CONCLUSIONS: SSTP had some short-term advantages over CAU, but not in the longer term. TRIAL REGISTRATION: Dutch Trial Register NTR2624. Registered 26 November 2010.