BACKGROUND: Infant-specific pertussis data, especially among neonates, are limited and variable. This study (NCT01890850) provides overall and age-specific pertussis incidence and associated health care utilization and costs among commercially insured infants in the US. METHODS: Nearly 1.2 million infants born from 2005 to 2010 with commercial health plan coverage were followed during their first 12 months of life. Pertussis cases were identified from medical claims (International Classification of Diseases, 9th revision, Clinical Modification code: 033.0, 033.9, 484.3), and incidence rates were calculated. Each pertussis case was then matched to 10 comparators, so pertussis-related health care utilization and costs before and after the index date could be assessed. RESULTS: The overall pertussis incidence rate among infants <12 months of age was 117.7/100,000 person-years; infants 3 months of age had the highest incidence rate (247.7/100,000 person-years). Infants diagnosed with pertussis were significantly more likely to have prior diagnoses of upper respiratory infection, cough and wheezing-related illnesses than comparators (P < 0.001). Pertussis cases were more likely to be hospitalized within 14 days after the index date (31.8% vs. 0.5%; P < 0.001) and their adjusted health care costs during follow-up were 2.82 times higher than comparators (P < 0.001; 95% confidence interval: 2.08-3.81). The incremental cost of pertussis during the 12-month follow-up period averaged $8271 (P < 0.001). The average incremental cost varied substantially by age, ranging from $18,781 (P < 0.001) to $3772 (P = 0.02) among infants 1 month and 7-12 months of age, respectively. CONCLUSIONS: The health burden of pertussis, particularly in the youngest infants, remains substantial, highlighting the need to intensify efforts to protect this most vulnerable population.