Low birthweight (LBW) (<2,500 g) is an important cause of infant morbidity and mortality.1 The rate of LBW has been steadily increasing in the United States. In 2005, the most recent year for which data are available, LBW represented 8.2% of all births, the highest level reported in the past 4 decades.2 The use of assisted reproductive technology (ART)* has been associated with LBW.3- 4 Research in 1999 indicated that, in Massachusetts during 1989-1996, the rate of LBW increased, paralleling the national trend, and an increasing percentage of LBW infants were born to mothers aged ≥35 years and to mothers with more education.† These findings suggested that a proportion of LBW births might be attributable to infertility treatment; however, at that time, no information was available from birth certificate records to examine whether ART was associated with the increasing rates of LBW. To investigate the role of ART, the Massachusetts Department of Public Health (MDPH) and CDC linked birth certificate records to ART records for the years 1997-2004 (the most recent data available). This report summarizes the results of that analysis, which indicated that, on average, 2% of births during the period resulted from ART; however, 7% of LBW births resulted from ART. The rate of LBW increased during this period among non-ART singletons (from 4.8% to 5.1%), accounting for an additional 407 LBW infants, and among ART singletons (from 6.4% to 8.2%), accounting for an additional 59 LBW infants. Although ART contributes disproportionately to LBW, only a small percentage of the excess LBW births in Massachusetts are explained by ART; therefore, other causes for the increase in LBW should be examined.