Since a defective ventilatory-control mechanism may have a role in the sudden-infant-death syndrome (SIDS), and hereditary factors influence the degree of ventilatory drive, we measured ventilatory responsiveness to carbon dioxide with and without increased airway resistance in 12 parents of SIDS victims and 12 control parents matched for age and size. Ventilatory response (delta VI/delta PCO2) and "respiratory drive" (delta P100/delta PCO2) were measured both with and without added resistance to inspiratory flow. SIDS parents had significantly lower ventilatory response with added resistance (P less than 0.05) and without it (P less than 0.01) and significantly lower respiratory drive with added resistance (P less than 0.001) and without it (P less than 0.05). Control parents had significantly increased respiratory drive when the resistance was added (P less than 0.005), whereas SIDS parents did not. The data suggest that a low ventilatory response to carbon dioxide and a diminished compensatory response to increased airway resistance may increase a potential parent's risk of having a child susceptible to SIDS.