Primary Sjögren's syndrome (pSS) is a very heterogeneous disease with systemic manifestations such as arthritis, skin, lung and renal involvement. To be able to assess systemic disease activity, the EULAR Sjögren's syndrome disease activity index (ESSDAI) was developed for use in daily clinical practice and in clinical trials. Since its development it has been widely used in cohort studies and clinical trials. The ESSDAI gives a systematic overview of a patient's systemic disease activity, which is very useful in daily clinical practice. However, using the ESSDAI as outcome measure in trials has been more challenging. Several RCTs with the ESSDAI as primary endpoint failed and showed large 'response rates' in placebo-treated patients as well. In this review, we discuss what we learned from using the ESSDAI in cohorts and clinical trials. We recommend to use the ESSDAI only in combination with other important outcome measures, such as patient-reported symptoms and glandular function as part of a composite endpoint in clinical trials in pSS patients.