Background/aims: The diagnostic properties of conventional diagnostic tests (ACE and chest radiography) for sarcoidosis-associated uveitis are not ideal. The diagnostic value of lymphopaenia for sarcoidosis-associated uveitis is investigated. Methods: A retrospective study of 191 consecutive patients with a first uveitis episode visiting the ophthalmology department (Erasmus Medical Center, Rotterdam, The Netherlands). Receiver operating characteristics (ROC) analysis was performed and compared with known ROC values from literature of conventional diagnostic tests for sarcoidosis-associated uveitis. An ideal cut-off was determined for lymphopaenia by calculation of the highest Youden index. Results: Out of all patients with first uveitis attack, 32/191 or 17% were subsequently diagnosed with biopsy-proven or radiological diagnosis of sarcoidosis. Lymphopaenia (<1.5×109/L) was significantly more often observed in patients with sarcoidosis-associated uveitis compared with patients with non-sarcoidosis-associated uveitis (p<0.05). The sensitivity and specificity of lymphopaenia was 75 % and 77 %, respectively. The optimal cut-off for lymphopaenia for diagnosing sarcoidosis-associated uveitis was 1.47 ×109/L. Lymphopaenia resulted in a 12.0 (95% CI 4.7 to 30.5 fold risk for having sarcoidosis, corrected for sex, race and age at onset of uveitis in patients with a first uveitis attack. Conclusion: Lymphopaenia is a non-invasive and useful marker for diagnosing sarcoidosis-associated uveitis.