One of the national health objectives for 2010 is to reduce the U.S. infant mortality rate (IMR) to </=4.5 deaths per 1,000 live births (objective no. 16-1c). Historically, Delaware's IMR exceeded the national average; however, during the early 1990s, Delaware's IMR decreased, reaching the U.S. average in 1993. During 1995--2000, the overall U.S. IMR decreased from 7.6 to 6.9; since 1996, Delaware's IMR has increased, and the state now has the seventh highest IMR nationally. To understand the cause of this increase, the Delaware Division of Public Health and CDC analyzed Delaware birth and death data. This report summarizes the results of the investigation, which attributed the Delaware IMR increase primarily to increased mortality among very low birthweight (VLBW) infants born to older, married, suburban mothers who were insured privately and who received early prenatal care. Further study, including collection of data on method of conception, is required to determine the etiology of the increased mortality in this subpopulation.