Increasing availability of antiretroviral treatment (ART) has led HIV to be considered a chronic disease, shifting attention to focus on quality of life including mental wellbeing. We investigated knowledge and causal attributions for mental disorders in HIV-positive children and adolescents in rural and urban Uganda. This qualitative study was nested in an epidemiological mental health study among HIV-positive children and adolescents aged 5–17 years in rural and urban Uganda. In-depth interviews were conducted with caregivers of HIV-positive children (5–11 years) and adolescents (12–17 years) in HIV care. Interviews were audio recorded with permission from participants and written consent and assent sought before study procedures. Thirty eight participants (19 caregivers, 19 children/adolescents) were interviewed. Age range of caregivers was 28–69 years; majority were female (17). Caregivers had little knowledge on mental disorders ;only 3 related the vignette to a mental problem and attributed it to: improper upbringing, violence, poverty and bereavement. Five adolescents identified vignettes as portraying mental disorders caused by: ill-health of parents, bereavement, child abuse, discrimination, HIV and poverty. Caregivers are not knowledgeable about behavioural and emotional challenges in HIV-positive children/adolescents. Mental health literacy programmes at HIV care clinics are essential to enhance treatment-seeking for mental health.