Respiratory syncytial virus infection was observed in 20 infants of less than 6 months of age who had a low weight at birth, were premature and/or presented with intra-uterine growth retardation. Except for rhinitis, which was present in all cases, the symptoms were closely correlated to the gestational age: beyond 48 weeks they were those of a common lower respiratory tract infection, but below 48 weeks attacks of apnea predominated, associated or not with signs of bronchitis. The apnea was of central origin, although there was no meningo-encephalitis; it regressed completely and without sequelae within a few days. In spite of similarities (notably of terrain) with disorders of respiratory command regarded as predictive of the sudden infant death syndrome risk, such children must be considered definitely cured after the initial episode. Including them in a long-term monitoring programme does not seem to be justified.