OBJECTIVE: to verify the major factors influencing weaning in preterm infants born in a public maternity center in the city of Sao Paulo, Brazil. METHOD: a total of 89 low-birth-weight preterm infants followed on an outpatient basis were studied from August 2006 to May 2007. These premature infants were divided into two groups according to the presence of partial weaning (introduction of supplementary bottle feeding) or full weaning (breastfeeding cessation). Maternal and infant variables obtained by means of interviews with the mothers and from the medical records were studied. The Student's t test and Spearman rank correlation test were used for the statistical analysis. The significance level was set at 5%. RESULTS: mean age for partial weaning was 1.41 months. Mothers who pumped during hospitalization or who started working or returned to work started bottle feeding later. The lower the birth weight and the gestational age, the later bottle feeding was started. The lower the age at pacifier introduction, the earlier the partial weaning. Mean age for full weaning was 2.93 months. Mothers hospitalized in the kangaroo mother care unit, or those who started working or returned to work, stopped breastfeeding later. FINAL CONSIDERATIONS: the findings of the present study point to the importance of factors that can be controlled by the health care team - such as breast milk pumping, the kangaroo mother care method, and advice not to use pacifiers - in the prevention of early weaning in preterm infants. Uncontrollable factors such as birth weight and maternal work were predictive of longer breastfeeding.