The case series method was developed by Farrington (1995) to investigate the strength of association between a time-varying exposure and an acute rare potentially recurrent event, using cases only. It can be used when the exposure can only be causally related to the event during a limited period of time. It has been widely used in pharmaco-epidemiology, particularly in the study of vaccine safety. The method is derived from a Poisson model by conditioning on the individual total number of events and its exposure history. As a consequence of this conditioning, the effects of fixed covariates cancel out, so that the method has a particular advantage as compared with cohort and case-control studies.