To determine the ability of physicians to make a diagnosis of pertussis and factors associated with improved diagnosis, 8,235 children from 88 child care centers and 14 elementary schools from Quebec City, Quebec, Canada, were evaluated by using a questionnaire completed by parents and a medical record review. Children must have consulted a physician to be included in the evaluation. There were 558 children meeting the surveillance case definition and 416 meeting a modified World Health Organization case definition who consulted a physician. A diagnosis of pertussis was considered in 24%–26% of children meeting either case definition, made in 12%–14%, and reported for 6%. Pertussis diagnosis was significantly associated with having a history of pertussis exposure (P ⩽ .003), four pertussis-related symptoms (P ⩽ .001), and a cough for ⩾5 weeks (P ⩽ .05) and consulting in a hospital setting (P ⩽ .03). The proportion of cases of pertussis diagnosed and reported is low even when children present with classical symptoms.