Multiple births (of all orders) increased in epidemic proportions in the years 1971-1997. Twins increased 53%, 32%, 31%, and 83% in white, Afro-American, Native American and Mexican American women, respectively. Triplet, quadruplet, and quintuplet+ births increased >400%, >1,100%, and >500%, respectively, in the same years. The principal causes of these changes are related to the increasing age of the maternal cohort and an increasing incidence of fertility-inhibiting diseases and conditions in association with advancing maternal age. Major immediate consequences of these changes include disproportionately large numbers of infants born at <33 weeks' gestation (1.7% for singletons vs. 41.2% for triplets) and at <1,500 g birth weight (1.1% for singletons vs. 31.9% for triplets). Additional short-term consequences include an almost 2,000% increase in infant deaths (per 1,000 live births) among triplets compared with singletons (190.4 vs. 11.2). Long-term risks include a 300% increase in the relative risk of handicap in triplets compared with singletons (2.9 vs. 1.0), and a 650% increase in the rate of cerebral palsy per 1,000 live births in triplets compared with singletons (26.6 vs. 1.6). Peripartum costs relate to prematurity rather than plurality, as do lifetime survivorship costs, which relate to morbidities and subsequent health-related problems.