During the course of a clinical study of Sotos syndrome, six out of 79 probands failed to fit the phenotype of Sotos syndrome but showed remarkable similarities to each other and to a further 11 first- and second-degree relatives. Clinical features in these index cases included macrocephaly, greater than +3.5 SD; normal or near normal birth weight and length with subsequent relative obesity; variable developmental delay; and typical face, characterised by a square outline with frontal bossing, a "dished-out" mid-face, biparietal narrowing, and long philtrum. All recorded bone ages were less than the 75th centile, and two were below the 10th centile. The authors believe the original diagnosis was incorrect and that these cases likely represent a previously undescribed autosomal dominant macrocephaly syndrome.