This study used patient discharge data from New Jersey to examine differences in length of stay for normal, uncomplicated deliveries between patients in health maintenance organizations (HMOs) and those not in HMOs. The percentage of one-day stays increased from less than 4% for all payers in 1990 to 48.1% for HMO patients, and 31.5% for non-HMO patients in 1994. Controlling for other factors, the odds of an HMO patient staying one day were nearly twice as great as a non-HMO patient by 1994; for all patients, regardless of payer, the odds of a one-day stay in 1994 were more than 18 times the odds of a one-day stay in 1990. The strong secular trend suggests that legislation and regulations should be targeted at particular policies rather than insurers.