OBJECTIVE The rate of primary cesarean delivery in Puerto Rico in 2002 was 52% higher than in 1996 and 85% higher than among Puerto Rican women delivering on the U.S. mainland. Reasons for these differences were explored using birth certificate data. METHODS Distributions of mothers' age, education, parity, level of prenatal care, pregnancy weight gain, medical risk factors, labor induction, labor or delivery complications, and infant birth weight among births in Puerto Rico in 2002 (n=40,489) were compared with births in Puerto Rico in 1996 (n=51,357) and births to Puerto Rican women delivering on the mainland in 2002 (n=47,800). Multivariable log-linear regression models were used to estimate relative risks for primary cesarean delivery by year, place of delivery, and selected risk factors. RESULTS Risk for cesarean delivery was higher in Puerto Rico in 2002 than in both 1996 (relative risk 2.1, 95% confidence interval 2.0, 2.3) and on the mainland in 2002 (relative risk 2.4, 95% confidence interval 2.2, 2.6). This translates into one additional cesarean delivery in Puerto Rico in 2002 for every 4.2 live births, controlled for examined risk factors. Higher rates of cesarean delivery in Puerto Rico in 2002 could not be explained by examined risk factors. CONCLUSIONS Until further research reveals ways to safely reduce the rate of cesarean delivery in Puerto Rico, physicians, public health practitioners, and other stakeholders may want to focus their efforts on reducing rates among low-risk women and those with no labor complications. METHODS II.