In the past decade, numerous sources have noted an increase in reported pertussis in highly immunized populations. This has been accompanied by a perceived change in disease epidemiology, characterized by a significant increase in reported pertussis incidence among adolescents and adults. In populations where children are routinely immunized, adolescents and adults now constitute the main source of infection in infants. However, a range of factors makes delineation of these epidemiologic trends difficult. Reported cases of pertussis represent only a fraction of the actual number of Bordetella pertussis symptomatic infections, because underconsulting, underrecognition and underdiagnosis are widespread and are a particular problem in adolescents and adults. Possible explanations for failure to diagnose pertussis include the heterogeneity in pertussis disease expression and low physician awareness and index of suspicion. Consequently defining pertussis from a clinical perspective is difficult, and this is reflected by a lack of consistency between case definitions. Although case definitions for specific circumstances have been established by the World Health Organization and the United States Centers for Disease Control, these are not universally useful, making intercountry comparisons and global evaluation difficult. Less-than-optimal and poorly performed laboratory tests, or their lack of availability, also make confirmation of B. pertussis infection difficult. To overcome these problems, clinical case definitions should be standardized for outbreak and endemic situations. Rapid, easy-to-use and inexpensive laboratory diagnostic techniques also must be made available and be widely implemented. In particular, polymerase chain reaction and single serum serology are 2 techniques that should be more widely adopted.