Patient-reported outcomes (PROs) are abundant in rheumatology and their numbers continue to increase. But which of the available measures are most important? Core outcome sets—including groups of domains and instruments for measuring them—have been defined for many rheumatic diseases, with the aim that all these outcomes should be measured in every clinical trial. The subgroup of PROs included in these core sets is, therefore, undoubtedly important. This Review summarizes the PROs included in core outcome sets developed for use in clinical trials across a wide range of rheumatic diseases. Three PROs are commonly utilized across the majority of rheumatic conditions: pain, physical functioning and the patient global assessment of disease activity. However, additional research is needed to fully understand the role of the patient global assessment of disease activity, to distinguish specific domains within the broad concept of health-related quality of life, and to work towards consensus on the choice between generic and disease-specific instruments in various contexts.