Although modern vaccines are usually well tolerated and efficacious, no vaccine is totally safe nor totally efficacious and adverse events have been reported following the use of each of them. International collaboration in the use, testing, and surveillance of vaccines would help limit these problems. National immunization programs must rely on comprehensive and timely surveillance and have the capability to react quickly to minimise negative outcomes. In each individual case, it may be difficult to demonstrate or rule out a relationship between a vaccine and an adverse event because there is usually no clear marker of etiology. Postmarketing surveillance of vaccines could be defined as the coordinated, structured, systematic, ongoing collection of data on the impact of licensed vaccines and their subsequent epidemiologic analysis and dissemination to manufacturers, regulators, health care providers, and the public. Managers of immunization programs have the responsibility for determining the risks associated with each vaccine, for constantly weighing the risks and benefits of vaccine use, and for minimizing the risks. Postmarketing surveillance of illness following administration of immunizing agents permits: 1) identification of illness or other side effects caused by immunizing agents; 2) estimation of the incidence of serious side effects of immunization; 3) monitoring for unusually high rates of adverse events (including lot-by-lot surveillance); 4) raising of health care providers' awareness to the risks and/or safety measures in administering vaccines; 5) identification of issues that require epidemiological research; and 6) identification of problems that require immediate epidemiological investigation.(ABSTRACT TRUNCATED AT 250 WORDS)