The fear of malpractice liability is mentioned frequently as a cause of increased cesarean section rates, but without quantitative investigations. This perception may be studied at an aggregate level by comparing malpractice insurance premiums, a proxy for liability risk, with primary cesarean section rates. Both New York and Illinois are divided into territories for insurance rates; the premium was uniform within each territory over the period studied for each specialty. Premiums for obstetricians were linked to birth and procedure data from New York and Illinois hospitals for 1981 and 1983, respectively, to determine whether there was a correlation between premium levels and the primary cesarean section rate. A statistically significant difference was found between mean cesarean rates by insurance premium territories in each State. A correlation was observed between increased insurance rates among territories and increased cesarean section rates. Based on these results, a substantial impact was found on delivery decisions resulting from the fear of malpractice suits.