Observational and longitudinal observational studies (LOS) provide essential information about the course and outcome of rheumatic disorders that cannot be provided by randomized controlled trials, and they constitute the major clinical scientific communication in rheumatology. There has been no consensus as to the full and appropriate content of LOS. This report defines a core set of domains and reporting requirements for LOS. At the 1998 OMERACT IV Conference a consensus process evaluated the literature of rheumatology in light of the constructs, variables, and outcomes of rheumatology by using introductory lectures, nominal groups, and plenary sessions. The result of this process was to identify 5 "core" domains that should be included in every LOS: Health Status, Disease Process, Damage, Mortality, and Toxicity/Adverse Reactions. Two additional domains, Work Disability and Costs, were recognized as important, but need not be used in all LOS. Eleven subdomains were identified that divided the domains into convenient clinical and conceptual units. A set of reporting requirements was also determined. The core recommendations, which follow on the WHO ICIDH-2 outline, are not disease-specific; the substitution of different "disease process" and "damage" measures make them suitable for many rheumatic disorders. The core set is intended to serve as a core for LOS in almost all rheumatic conditions.