objective. to study the association between maternal working conditions and low birth-weight in mexico city. material and methods. interviews of 2 623 workers who gave birth in mexico city hospitals during 1992 were analyzed. information on the main biologic and social factors associated to low birth-weight was registered. occupational stress was determined with the instrument designed by karasek. logistic regression models to evaluate the relationship between working conditions and low birth-weight were used, controlling by confounding variables. results. low birth-weight was more frequent in workers with working periods of more than 50 h/week (or= 1.6; 95% ci= 1.17, 2.28) and with problems at work (or= 1.5; 95% ci= 1.0, 2.25). lack of tangible social support was identified as a risk factor for low birth-weight (or= 1.7; 95% ci= 1.20, 2.33). preventive working measures such as changes in tasks, shortening of working hours and leaves of absence due to illness did not show a beneficial effects on birth-weight, except for the maternity leave of absence. mothers with no right to this had a 2.2 higher probability of giving birth to low weight children (95% ci= 1.66, 2.93). conclusions. these results emphasize the importance of identifying the occupational risk factors during pregnancy.