We describe two siblings with Sotos syndrome, probably inherited from their father. The diagnosis was made early in both children, who presented psychomotor delay from early on as well as a strikingly similar phenotype (macrocrania, coarse face, accelerated growth). The prolonged follow-up of both children shows severe but non progressive motor and language developmental delay, followed by improvement with age in the girl. This improvement is also expected for the boy, who shows identical development. The expressive language delay present in both children may be explained by oral apraxia rather than mental handicap. The progressive motor and mental improvement, which can occasionally reach normal level in adults, and the tendency toward phenotypic normalization could explain the difficulty in making a definite diagnosis in adults and thus in defining the pattern of inheritance.