Prader-Willi syndrome (PWS) is a neurogenetic disorder with hypotonia, psychomotor delay, obesity, short stature, and sleep-related breathing disorders. The aim of this study was to evaluate the association between psychomotor development and sleep-related breathing disorders in PWS infants. Bayley Scales of Infant Development were performed in 22 PWS infants, with a median (interquartile range, IQR) age of 1.8 (1.1-3.4) y, and a body mass index SD score (BMISDS) of -0.5 (-1.3 to 1.6). We evaluated psychomotor development in relation to results of polysomnography. Median (IQR) mental and motor development was 73.1% (64.3-79.6%) and 55.2% (46.5-63.1%) of normal children, respectively. All infants had sleep-related breathing disorders, mostly of central origin. The apnea hypopnea index was not associated with psychomotor development. Only four infants had obstructive sleep apnea syndrome (OSAS). They had a significantly delayed mental development of 65.5% (60.0-70.3%) of normal. They had a median BMISDS of 1.4 (0.1-1.6), which tended to be higher than in those without OSAS. Our data indicate that psychomotor development in PWS infants is not related to central sleep-related breathing disorders, but infants with OSAS have more severely delayed mental development, suggesting that PWS infants should be screened for OSAS.