The GAVI Alliance – an alliance of the world’s major players in global immunisation – uncovered massive misuse of its grants in Cameroon in 2011. GAVI’s Transparency and Accountability Policy triggered an investigation into the financial management of Cameroon’s Health Systems Strengthening grant. The investigation revealed that, of US$5.1 million programme expenditures, US$3.7 million had been misspent, partly due to fraud. Different types of corruption affected this programme. Analysing how misuse was detected and how GAVI and the government responded, provides lessons on how similar abuses can be prevented in the future. In order to avoid grant mismanagement, programmes should have integrated transparency and accountability systems. This is also applicable to programmes in other sectors.