This article discusses the challenges of estimating levels and patterns of heroin use in a setting where there were no official records. Ethnographic fieldwork, carried out in a Kenyan Coastal town, utilised a range of qualitative research methods in an attempt to estimate numbers of male and female users and the proportion of them who were injectors of heroin. In the town of at least 85 000 people, it was estimated that there were perhaps about 600 heroin users, of whom about 30 were women. The ratio of male to female users was estimated to be 20 : 1. Fifty per cent of users in the town were estimated to be injectors of heroin. They were found to have poor injecting techniques, to share equipment from time to time and to have low awareness of the link between injecting drug use and HIV infection. An urgent need for harm reduction strategies was identified.