Referrals for deformational plagiocephaly (DP) have increased, but estimates of its actual prevalence vary, depending on the population studied and criteria for diagnosis. Few studies employ an objective technique for diagnosis. The objectives of this study were to validate the Transcranial Diameter Difference (TDD) and, using it, determine the prevalence of DP among infants seen by primary care pediatricians. We determined the TDD of 32 infants referred to a craniofacial clinic for DP; blinded to the TDD a craniofacial surgeon assigned a DP severity score. We compared the TDD and severity scores. The TDD of 192 infants presenting to primary care practices (PCP) were determined and their parents completed a DP risk factor questionnaire. Odds ratios for associations of risk factors with DP were calculated. The correlation between TDD and DP severity score was 0.61 (P = 0.002). All infants whose TDD > 0.6 cm had a severity score > 2; 18.2% of the 192 infants had DP as defined by a TDD > 0.6 cm. Significant odds ratios (95% confidence intervals) for the presence of DP were sleeping supine, 3.5; (1.6, 7.5), and infant head position preference 2.2; (1.0, 4.9). Varying the sleep position decreased the risk of DP, OR = 0.40 (0.2, 0.9). We conclude that the TDD is a valid, objective measure of DP for use in research studies. DP is present in nearly one in five PCP infants. Because an infant who prefers to hold his head in one position is more likely to have DP, advising parents to vary the head position may reduce the risk of DP.