IMPORTANCE: Undervaccination is an increasing trend that potentially places children and their communities at an increased risk for serious infectious diseases. OBJECTIVE: To examine the association between undervaccination and pertussis in children 3 to 36 months of age. DESIGN: Matched case-control study with conditional logistic regression analysis. SETTING: Eight managed care organizations of the Vaccine Safety Datalink between 2004 and 2010. PARTICIPANTS: Each laboratory-confirmed case of pertussis (72 patients) was matched to 4 randomly selected controls (for a total of 288 controls). The case patients were matched to controls by managed care organization site, sex, and age at the index date. The index date was defined as the date of pertussis diagnosis for the case patients. EXPOSURE: Undervaccination for the diphtheria, tetanus toxoids, and acellular pertussis (DTaP) vaccine. Undervaccination was defined as the number of doses of DTaP vaccine that was either missing or delayed by the index date. Case patients and controls could be undervaccinated by 0, 1, 2, 3, or 4 doses of DTaP vaccine. Children undervaccinated by 0 doses were considered age-appropriately vaccinated by the index date. MAIN OUTCOME AND MEASURE: Pertussis. RESULTS: Of the 72 case patients with pertussis, 12 (16.67%) were hospitalized, and 34 (47.22%) were undervaccinated for DTaP vaccine by the date of pertussis diagnosis. Of the 288 matched controls, 64 (22.22%) were undervaccinated for DTaP vaccine. Undervaccination was strongly associated with pertussis. Children undervaccinated for 3 or 4 doses of DTaP vaccine were 18.56 (95% CI, 4.92-69.95) and 28.38 (95% CI, 3.19-252.63) times more likely, respectively, to have received a diagnosis of pertussis than children who were age-appropriately vaccinated. CONCLUSIONS AND RELEVANCE: Undervaccination with DTaP vaccine increases the risk of pertussis among children 3 to 36 months of age.