INTRODUCTION: the low birth weight (LBW) is considered one of the most important public health problems around the world, contributing substantially to infantile morbidity and mortality. OBJECTIVES: to estimate the proportion of low birth weight and identify the factors associated. METHOD: cross-sectional study which examined 3220 statements of births relating to births occurring in the city of Cruzeiro do Sul, Acre, in the period 2006 to 2007, of resident mothers in this locality. In the analysis, linear regression generalized with Poisson family logarithmic linking robust variance, simple and multiple was used, with a significance level of 0.10. RESULTS: the proportion of low birth weight was 9.13%. Factors associated with low birth weight were: prematurity, birth at home, female, maternal age between 12 and 13 years, 16 and 17 years, 18 and 19 years, more than 34 years, achieving 1-3 pre-natal consultations, non-white children, mothers with no occupation outside home and single mothers. CONCLUSION: few (or none) factors found are susceptible to change or control with isolated health actions. Wide variety of strategies are needed to reduce the proportion of LBW in the city of Cruzeiro do Sul, Acre, and when low birth weight occurred attention should be given to the child.