This article assesses whether and how investigators are monitoring adverse events following immunization (AEFI) in vaccine trials using evidence from the WHO Vaccine Trial Registry. It is noted that the Registry includes all vaccine trials sponsored since 1987 by the WHO Expanded Programme on Immunization Global Programme for Vaccines and Immunization and Department of Vaccines and Biologicals. For each trial records include internal documents reports of visits to trial sites and publications. Based on the records from 68 trials completed or in progress analysis indicates that only few investigators included detailed AEFI monitoring in their study reports and publications. However an increasing trend to include AEFI monitoring in vaccine clinical trials was noted. Since many vaccine trials are conducted by independent investigators and AEFI monitoring methods and results deserve to be included in any publication along with vaccine efficacy methods and results it should be the responsibility of the study investigators rather than of the vaccine manufacturer and the national control authority as suggested. Several practical points for monitoring AEFIs in vaccine clinical trials are cited.