Using a historical case study of the World Bank and World Health Organization (WHO)'s Onchocerciasis Control Programme (OCP, 1972-2002), I explore how success is conceptualised in global health and why it matters for policy and priority-setting. First, I summarise the 'dominant' OCP success narrative that has emerged since the 1980s, which is based on public health, socio-economic and humanitarian justifications for the programme's effectiveness. Next, I analyse how socio-economic metrics linking the programme's disease control to increased labour productivity and agricultural land availability evolved in the 1980-90s. This alternative analysis of the OCP demonstrates how metrics, particularly when divorced from their assumptions and political context, are pliable and constructible. I argue that the OCP's success was actively constructed by the World Bank and that moving beyond triumphalist, programme-level 'lessons-learned' approaches within global health requires disruption of the epistemic, institutional and discursive power that 'lies beneath' success narratives.