BACKGROUND: Increasing populations of children who are HIV-exposed but uninfected will face the challenge of disclosure of parental HIV infection status. We aimed to test the efficacy of an intervention to increase maternal HIV-disclosure to primary school-aged HIV-uninfected children. METHODS: This randomised controlled trial was done at the Africa Health Research Institute in KwaZulu-Natal, South Africa. Women who had tested HIV positive at least 6 months prior, had initiated HIV treatment or been enrolled in pretreatment HIV care, and had an HIV-uninfected child (aged 6-10 years) were randomly allocated to either the Amagugu intervention or enhanced standard of care, using a computerised algorithm based on simple randomisation and equal probabilities of being assigned to each group. Lay counsellors delivered the Amagugu intervention, which included six home-based counselling sessions of 1-2 h and materials and activities to support HIV disclosure and parent-led health promotion. The enhanced standard of care included one clinic-based counselling session. Outcome measures at 3 months, 6 months, and 9 months post baseline were done by follow-up assessors who were masked to participants' group and counsellor allocation. The primary outcome was maternal HIV disclosure (full [using the word HIV], partial [using the word virus], or none) at 9 months post baseline. We did the analysis in the intention-to-treat population. This study is registered with ClinicalTrials.gov (NCT01922882). FINDINGS: Between July 1, 2013, and Dec 31, 2014, we randomly assigned 464 participants to the Amagugu intervention (n=235) or enhanced standard of care (n=229). 428 (92%) participants completed the 9 month assessment by Sept 3, 2015. Disclosure at any level was more common in the Amagugu intervention group (n=204 [87%]) than in the enhanced standard-of-care group (n=128 [56%]; adjusted odds ratio 9.88, 95% CI 5.55-17.57; p<0.0001). Full disclosure was also more common in the Amagugu intervention group (n=150 [64%]) than in the enhanced standard-of-care group (n=98 [43%]; 4.13, 2.80-6.11; p<0.0001). Treatment-unrelated adverse effects were reported for 17 participants in the Amagugu intervention group versus six in the enhanced standard-of-care group; adverse effects included domestic violence (five [2%] in the Amagugu intervention group vs one [<1%] in the enhanced standard-of-care group), sexual assault (four [2%] vs one [<1%]), participant illness or death (four [2%] vs four [2%]), and family member illness or death (four [2%] vs none). No treatment-related deaths occurred. INTERPRETATION: The lay-counsellor-driven Amagugu intervention to aid parental disclosure has potential for wide-scale implementation after further effectiveness research and could be adapted to other target populations and other diseases. Further follow-up and effectiveness research is required. FUNDING: National Institutes of Health.