Aims To assess the influence of household decision making, social capital, socio-economic factors and health service use on under-five mortality. Setting Butajira Demographic Surveillance Site, Ethiopia. Methods A prospective case-referent design with a total of 209 under-five year old deaths occurring in an 18-month period, together with 627 referents matched for age, sex and community of residence were included. Questionnaires were administered to mothers or caretakers. Matched case control analysis investigated the effect of risk factors on mortality and the presence of avoidable factors was assessed for each death. Results Lack of immunization was strongly associated with mortality (adjusted OR = 9.8, 95% CI 5.9, 16.1). Low decision making capacity of women (adjusted OR= 3.2 95% CI 2.0, 5.0) and low social capital scores (adjusted OR = 1.9 95% CI 1.1, 3.5) were also related to high under-five mortality in multivariate analyses. Potentially avoidable household and health facility factors were identified, respectively in 71% and 40% of the deaths. Conclusion Combined efforts to improve women's involvement in household decision making, social capital and immunization may decrease the high child mortality in this setting where the level of poverty is high and no appreciable trend in child mortality decline has been noted over the years.