The rate of delivery by cesarean has increased steadily in the United States since the 1970s. The reasons for this increase are not fully established. Improved diagnosis of maternal and fetal complications, medicolegal concerns, and the changing age composition of childbearing women have been cited as contributing factors. To assess whether advanced maternal age by itself is an indicator for a primary cesarean delivery, we analyzed data from the vital records of all female residents of King County, Washington 35 years and older (N = 2985) who had a live singleton birth in 1986 or 1987. These women were compared with a sample of women 20-29 years old (N = 6140) who gave birth in the same time period and geographic area. Primiparous and multiparous women 35 years and older were at a similar increased risk of cesarean (relative risk = 1.6, 95% confidence interval 1.1-2.4). Primiparous women of all ages experienced more complications of pregnancy and labor and higher cesarean delivery rates. However, among primiparous mothers with no recorded complications, older women were at significantly increased risk of cesarean birth (relative risk = 2.5, 95% confidence interval 1.8-3.5). This analysis suggests that advanced maternal age alone may influence a physician's decision regarding method of delivery, thereby placing some older women at an unnecessary risk of cesarean.