Background In the past 15 years, Brazil has undergone notable social and public health changes, including a large reduction in child mortality. The Bolsa Familia Programme (BFP) is a widespread conditional cash transfer programme, launched in 2003, which transfers cash to poor households (maximum income US$70 per person a month) when they comply with conditions related to health and education. Transfers range from $18 to $175 per month, depending on the income and composition of the family. We aimed to assess the eff ect of the BFP on deaths of children younger than 5 years (under-5), overall and resulting from specifi c causes associated with poverty: malnutrition, diarrhoea, and lower respiratory infections. Methods The study had a mixed ecological design. It covered the period from 2004–09 and included 2853 (of 5565) municipalities with death and livebirth statistics of adequate quality. We used government sources to calculate allcause under-5 mortality rates and under-5 mortality rates for selected causes. BFP coverage was classifi ed as low (0·0–17·1%), intermediate (17·2–32·0%), high (>32·0%), or consolidated (>32·0% and target population coverage ≥100% for at least 4 years). We did multivariable regression analyses of panel data with fixed-eff ects negative binomial models, adjusted for relevant social and economic covariates, and for the eff ect of the largest primary health-care scheme in the country (Family Health Programme). Findings Under-5 mortality rate, overall and resulting from poverty-related causes, decreased as BFP coverage increased. The rate ratios (RR) for the eff ect of the BFP on overall under-5 mortality rate were 0·94 (95% CI 0·92–0·96) for intermediate coverage, 0·88 (0·85–0·91) for high coverage, and 0·83 (0·79–0·88) for consolidated coverage. The eff ect of consolidated BFP coverage was highest on under-5 mortality resulting from malnutrition (RR 0·35; 95% CI 0·24–0·50) and diarrhoea (0·47; 0·37–0·61).