Multiple risk factors for deformational plagiocephaly (DP) have been reported. The purpose of this study was to establish the impact of these variables on the severity of this deformity. A prospective cohort study was performed. Parents completed a standardized questionnaire assessing potential risk factors for DP before assessment. Examination included measurement of transcranial difference (TCD; ie, difference in oblique cranial lengths), evaluation of head tilt, and rotational asymmetry. Pearson correlation coefficient, 1-way analysis of variance, and 2-sample t-test were used to quantify the relationship between identified risk factors and TCD. A total of 434 patients with DP were evaluated. Male-to-female ratio was 2:1; mean gestational age was 36.5 weeks. Deformational plagiocephaly was first appreciated at a mean infant age of 6 weeks. A preexisting diagnosis of torticollis was noted in fewer than 50%. Mean TCD was 11.2 mm. Head tilt was documented in 80% of infants, and mean head rotational asymmetry was 16.4 degrees. Deformational plagiocephaly was more severe in multiple birth pregnancies (P < 0.05), males (P < 0.05), infants with a favorite head position (P < 0.01), preexamination diagnosis of torticollis (P < 0.05), and infants with a head tilt (P < 0.05). Lower gestational age (P < 0.05) and greater head rotational asymmetry (P < 0.0001) were found to correlate with DP severity. This study suggests that the relationship between the severity of DP and certain risk factors can be quantified. The presence and degree of cervical imbalance correlate strongly with deformational cranial asymmetry.