In the Swiss Sentinel System pertussis is reported according to a clinical case definition. In our Sentinel practice, we have investigated in detail nine patients fulfilling these criteria. Seven of the nine patients have previously been vaccinated with whole cell vaccine. The clinical diagnosis could be confirmed in three cases by the detection of elevated serum IgA to filamentous hemagglutinin (FHA), and in one infant based on the peripheral blood lymphocytosis and the detection of pertussis in other family members. The likely diagnosis in another brother and sister pair was a parainfluenza virus infection, possibly with concomitant pertussis. Two sisters probably had a mycoplasmal infection, which in one was accompanied by pertussis. In two patients an infectious etiology could not be determined. The laboratory tests which have been used in these cases are discussed. The occurrence of pertussis in previously vaccinated adolescents and adults has been recognized worldwide and is of epidemiological concern in view of current vaccination policies. The Swiss Sentinel System could play an important role in the surveillance of pertussis in Switzerland.