Because young infants are at highest risk for severe pertussis and death and are also too young to have received the minimal protective series of three doses of diphtheria-tetanus-pertussis (DTP) vaccine, we conducted a matched case-control study to assess risk factors for pertussis among young infants during a pertussis outbreak in Chicago in 1993. We enrolled 39 cases < 7 months of age from a single teaching hospital and 96 controls, individually matched for age, from the well-child clinic at the same hospital. Demographic characteristics, immunization status, and opportunities for disease exposure were analyzed by means of conditional logistic regression. Cases and controls were similarly up to date with their DTP vaccinations (87% and 89%, respectively). Infants of adolescent mothers (matched odds ratio [OR], 6.4; 95% confidence interval [CI], 1.3-41.4) and infants of mothers who suffered > or = 7 days of cough during the child's incubation period (matched OR, 12.0; 95% CI, 1.4 to infinity) were significantly more likely to have pertussis. Young mothers and mothers with a cough lasting > or = 7 days may be an important source of pertussis infection for their young infants.