Diagnosis of pertussis by culture and PCR is most sensitive when performed on nasopharyngeal specimens collected <2 weeks and <3 weeks, respectively, after the onset of clinical disease. Conversely, serological testing allows the diagnosis of patients (mostly adults) with less typical whooping cough symptoms, for whom clinical samples are often collected at later time points. Here, we report on a 20-year serodiagnostic survey of pertussis in Belgium from 1990 to 2009. In total, 13,163 patients were analyzed for Bordetella pertussis-specific antibodies by agglutination, complement fixation, immunofluorescence, and ELISA. The number of positive pertussis cases detected by serodiagnosis ranged between 50 and 150 annually. The mean age of positive cases increased from 9.9 years in 1990 to 33.9 years in 2009. Whereas from 1990 to 2003, children and young adolescents made up the majority of cases, from 2004 onwards, cases were detected in all age groups and the distribution became bimodal, with a first peak at the age of 10 to 20 years and a second at the age of 35 to 50 years. In contrast, patients diagnosed since 2001 by PCR and/or culture were mostly children younger than 1 year of age. Despite extensive childhood vaccination campaigns, whooping cough is still present in Belgium. Our findings confirm the potential role of adults in the continued transmission of pertussis and strongly warrant booster or cocoon vaccinations in older age groups.