In a study of 2387 employed women who had worked for more than three months of their pregnancy the data were extracted from a survey carried out on a national sample of births in France in 1981. Manual, service and shop workers had a higher preterm delivery rate than professional, administrative, or clerical workers. Assembly line work was associated with a higher preterm delivery rate even when production workers only were considered. Cumulated physically tiring working conditions--standing work, carrying of heavy loads, assembly line work, and considerable physical effort--were related to higher preterm delivery and low birthweight rate. During pregnancy, sickness absences were commoner when the working conditions were arduous. Changes in the working conditions were less clearly related to arduous work than sick leaves; they were not significantly more frequent for standing work or for assembly line work. Refusals from employers to grant favourable arrangements were more frequent when the working conditions were tiring and sick leaves were more common among women whose requests had been refused.