ABSTRACT. Fourteen children with the Prader‐Wllli syndrome have been managed at the Royal Alexandra Hospital for Children between the years 1964–1980 — twelve male, two female. Six male children developed features of the obesity hypoventilation syndrome. The age of onset of this complication ranged from 4.0 to 12.6 years. With one exception those children with the obesity hypoventilation syndrome were more obese than those without it. At the time of onset of the syndrome, five of six patients had weights greater than or equal to 6.5 standard deviations above ideal body weight. Those children without the obesity hypoventilation syndrome had a range of standard deviations 1.0 to 4.2 above the ideal body weight. In four of six cases weight reduction and a cardiac failure regimen resulted in reversal of the obesity hypoventilation syndrome. With two of the six children there had been cardiomegaly and increased pulmonary venous vascularity on x‐ray at a chronological age of three months. Two of the six children died.