In August 1991 the Institute of Medicine released a report entitled "Adverse Effects of Pertussis and Rubella Vaccines" that examined, among other relations, the relation between immunization with the RA 27/3 rubella vaccine strain and chronic arthritis. The committee spent 20 months reviewing a wide range of information sources including case series and individual case reports published in peer-reviewed journals and reported by vaccine manufacturers; unpublished case reports from physicians, parents, and other concerned persons; epidemiological studies; and laboratory studies. There were no animal studies available. The committee found that the evidence is consistent with a causal relation between the RA 27/3 rubella vaccine strain and chronic arthritis in adult women, although the evidence is limited in scope. Proving that rubella vaccination can cause chronic arthritis will require a better understanding of pathogenetic mechanisms and additional well-designed studies. We briefly describe the committee's evaluative methods and present the evidence underlying its conclusion.