Objective: To examine factors that may explain a 9-fold increase in plagiocephaly in Texas from 1999 to 2007., Design: Descriptive epidemiologic study of time trends and a nested case-control study., Setting: Texas., Participants: Cases in the Texas Birth Defects Registry., Outcome Measures: Time trends in the birth prevalence of plagiocephaly overall and by region, demographic group, and clinical subgroup. Trends in percentage of cases using specific facilities or procedures., Results: From 1999 to 2007, the prevalence of plagiocephaly in Texas increased from 3.0 cases per 10 000 live births to 28.8, an average increase of 21.2% per year. This was highly statistically significant. The time trend was most pronounced in the Dallas/Fort Worth region and in certain health care facilities. It was observed in all demographic and clinical subgroups. Cases born in 2004 and 2005 were not more likely to be postnatally acquired when compared with cases born in 1999 and 2000. There was no commensurate decrease in other birth defects of the face or skull., Conclusions: A small part of the trend might have been due to delayed compliance with the infant supine sleeping recommendation and a slight increase in preterm births. It was not due to changes in birth defect coding practice or trends in multiple births, torticollis, or oligohydramnios. Because the plagiocephaly trend was observed mainly in patients visiting certain health care facilities, among mild cases, and among cases with minimally invasive procedures, we suspect it may be due mainly to changes in available therapies and insurance reimbursement practices.