Occipital plagiocephaly is characterized by both unilateral occipital flattening and ipsilateral frontal prominence with anterior deviation of the ipsilateral ear, yielding a characteristic parallelogram shape to the cranium. Radiographic changes in the lambdoid suture are often evident, but the lambdoid suture is usually patent over most or all of its length on skull X-rays and/or CT scans. Both lambdoid synostosis and deformational forces have been implicated as potentially causal in the pathogenesis of this deformity. We proposed a unifying theory which incorporates a common pathogenesis for both deformational plagiocephaly and most cases of lambdoid 'synostosis'. According to this hypothesis, intrauterine and/or postnatal deformation forces are responsible for the primary calvarial deformation. These forces initially act in reversible manner to produce the typical parallelogram-shaped skull deformity. However, with continued deformation, more enduring secondary pathological changes may eventually occur in the lambdoid suture and basicranium which are more difficult to correct even if the offending deformational forces are subsequently removed or reversed.