We conducted a community-based nested case-control study of post-neonatal deaths in children under 5 years, with frequency-matched controls chosen from a full sampling frame provided by a demographic surveillance system. Using a questionnaire, we studied treatment-seeking behaviour in fatal illness. In cases and controls we investigated demographic and socio-economic factors, health-seeking behaviour, the household environment including accessibility of health care, and individual child care factors. Half of the deaths (215/427) occurred at home, and one-third (146/427) at a health facility. Three-quarters (330/427) of the children who died had received treatment from a health facility in their fatal illness. Four independent risk factors for death were identified which were each associated with more than 5% of child mortality: not being carried on the back while the mother cooked [odds ratio (OR) 1.6: 1.3, 2.0], poor maternal education (OR 1.4: 95% CI 1.0, 1.9 for those with no education compared with those with complete primary education), lack of exclusive breastfeeding in the first 3 months of life (OR 1.4: 1.1, 1.8), and low socio-economic status (OR 1.3: 1.0, 1.6). The majority of children who died had sought treatment at a health facility during the fatal illness, which would lend support to interventions to improve case-management. The association between mothers carrying their children and child survival was unexpected and deserves further investigation.