In the Senegal pertussis trial, common adverse reactions were actively monitored during the pilot phase II study, while the frequency of severe adverse reactions was monitored as a secondary objective within the phase III efficacy trial. Since the trial was conducted in Niakhar, an area in rural West Africa under intensive surveillance, the safety monitoring during the study was incorporated within the general surveillance system. This was a two-step procedure: detection of a potential reaction by a field worker, followed by confirmation report by a physician. The frequency of severe reactions was low among both pertussis vaccine groups, receiving either the two-component acellular vaccine or the whole-cell vaccine currently used in the Senegal Expanded Programme on immunisation. Among severe reactions, only persistent crying was found to be at a significantly higher rate in the whole-cell group. Common adverse reactions were more frequent in the whole-cell group.