In a multicenter, randomized, placebo-controlled clinical trial of pertussis immune globulin, intravenous (P-IGIV), 25 infants hospitalized with pertussis were enrolled in 24 months (15% of the target sample size) before the study was prematurely terminated because of expiration of the P-IGIV lots and unavailability of additional study product. Although well tolerated, there was no difference in the number or rate of improvement of symptoms (paroxysmal cough, whoop, apnea, bradycardia, oxygen desaturations) in P-IGIV recipients compared with placebo.