BACKGROUND: Poor early childhood development (ECD) in low- and middle-income countries is a major concern. There are calls to universalise access to ECD interventions through integrating them into existing government services but little evidence on the medium- or long-term effects of such scalable models. We previously showed that a psychosocial stimulation (PS) intervention integrated into a cash transfer programme improved Colombian children's cognition, receptive language, and home stimulation. In this follow-up study, we assessed the medium-term impacts of the intervention, 2 years after it ended, on children's cognition, language, school readiness, executive function, and behaviour. METHODS AND FINDINGS: Study participants were 1,419 children aged 12-24 months at baseline from beneficiary households of the cash transfer programme, living in 96 Colombian towns. The original cluster randomised controlled trial (2009-2011) randomly allocated the towns to control (N = 24, n = 349), PS (N = 24, n = 357), multiple micronutrient (MN) supplementation (N = 24, n = 354), and combined PS and MN (N = 24, n = 359). Interventions lasted 18 months. In this study (26 September 2013 to 11 January 2014), we assessed impacts on cognition, language, school readiness, executive function, and behaviour 2 years after intervention, at ages 4.5-5.5 years. Testers, but not participants, were blinded to treatment allocation. Analysis was on an intent-to-treat basis. We reassessed 88.5% of the children in the original study (n = 1,256). Factor analysis of test scores yielded 2 factors: cognitive (cognition, language, school readiness, executive function) and behavioural. We found no effect of the interventions after 2 years on the cognitive factor (PS: -0.031 SD, 95% CI -0.229-0.167; MN: -0.042 SD, 95% CI -0.249-0.164; PS and MN: -0.111 SD, 95% CI -0.311-0.089), the behavioural factor (PS: 0.013 SD, 95% CI -0.172-0.198; MN: 0.071 SD, 95% CI -0.115-0.258; PS and MN: 0.062 SD, 95% CI -0.115-0.239), or home stimulation. Study limitations include that behavioural development was measured through maternal report and that very small effects may have been missed, despite the large sample size. CONCLUSIONS: We found no evidence that a scalable PS intervention benefited children's development 2 years after it ended. It is possible that the initial effects on child development were too small to be sustained or that the lack of continued impact on home stimulation contributed to fade out. Both are likely related to compromises in implementation when going to scale and suggest one should not extrapolate from medium-term effects of small efficacy trials to scalable interventions. Understanding the salient differences between small efficacy trials and scaled-up versions will be key to making ECD interventions effective tools for policymakers. TRIAL REGISTRATION: ISRCTN18991160.