Appropriate management of posterior plagiocephaly requires differentiation of occipitoparietal flattening caused by lambdoid synostosis from that caused by deformational forces. In a 2 1/2-year prospective study of 115 infants presenting with unilateral posterior cranial flattening, only one child had synostotic posterior plagiocephaly (lambdoid synostosis), whereas 114 infants had deformational posterior plagiocephaly. Deformational occipitoparietal flattening was more common on the right (61 percent) than on the left (30 percent), and minor contralateral frontal flattening was not unusual (52 percent). The ipsilateral ear was anteriorly displaced in virtually all infants (97 percent). Some infants had ipsilateral torticollis (19 percent); a few had contralateral torticollis (8 percent). Gender ratio was 3:1, male:female. A total of 114 infants with deformational posterior plagiocephaly were treated conservatively either by head positioning in the crib (n = 63) or with a molding helmet (n = 51). Outcome was assessed by pretreatment and posttreatment anthropometry on 53 of these infants, who were either positioned (n = 17) or helmeted (n = 36). Improvement occurred in 52 of 53 patients (mean follow-up 4.6 months), i.e., the difference in length between the long and short transcranial axis diminished in 52 infants (mean 1.2 to 0.7 cm), did not progress in any child, and was unchanged in one infant. At an average age of 10 months, posterior cranial symmetry was better in infants treated with a helmet (mean difference 0.6 cm) than in those managed by positioning (mean difference 1.0 cm) (p < 0.001). Age at initiation of helmet therapy (from 2 to 9 months) was unrelated to rate of improvement. In a 10-year retrospective study, the authors identified 12 infants who had an operation for posterior plagiocephaly. All but one had confirmed premature lambdoid fusion; thus, this condition accounted for 3.4 percent of all primary operations performed for craniosynostosis during this decade (n = 323). In retrospect, the physical findings of synostotic posterior plagiocephaly were not clearly different from those of deformational posterior plagiocephaly. Plain radiography was sometimes used to confirm the clinical diagnosis. Neither sutural narrowing, deep interdigitations, nor perisutural sclerosis indicated lambdoid synostosis. Computed tomography (CT) was necessary if the physical findings were suspicious for lambdoid synostosis or if plain films did not give a definitive diagnosis. Axial CT scans (n = 7) showed a symmetric forehead in all but one patient with lambdoid synostosis. CT studies also demonstrated that auricular position was indeterminate in synostotic posterior plagiocephaly, being anterior, posterior, or symmetric, whereas the ipsilateral ear was virtually always anterior in deformational posterior plagiocephaly.