Respiratory failure has been associated with depressed ventilatory responses to hypoxia or hypercapnia or both. The possibility that familial factors are responsible for decreased chemosensitivity prompted this study of a child with unexplained respiratory failure and normal lung function. We found his ventilatory response to hypoxia and hypercapnia to be virtually absent. Studies of six healthy immediate family members (parents and siblings) showed that hypoxic response, as measured by an index of the relation between ventilation and hypoxia (index A), was consistently reduced: 45 +/- 8.7 S.E.M. (normal, 127 +/- 8.7) (P less than 0.005). Response to hypercapnia, measured as the slope of the ventilatory response to hypercapnia, was lower than normal, averaging 0.95 +/- 0.16 liters per minute per millimeter of mercury (normal, 1.76 +/- 0.13) (P less than 0.01). The patient's respiratory failure seemed related to deficient ventilatory responses to hypoxia and hypercapnia. It seems likely that this depressed hypoxic response is of familial origin.