The purpose of this study is to evaluate the possibility of using the mucus ferning phenomenon of the tears as a diagnostic test for Keratoconjunctivitis sicca in patients with Sjögren's syndrome. Using a polarising light microscope dried samples were tested for ferning phenomenon collected from (1) 36 healthy controls, (2) 21 patients with primary Sjögren's syndrome, and (3) 15 patients with secondary Sjögren's syndrome. Patients with Sjögren's had xerostomia, abnormal salivary gland biopsy and at least two positive tests for Kerato-conjunctivitis sicca. Patterns of mucus crystallization were classified into 5 types according to uniformity, branch spreading and integrity (types I, II normal, types III, IV, V abnormal). Abnormal ferning was found in tear samples of 8 out of 72 normal control eyes, 38 out of 42 with primary Sjögren's syndrome and 25 out of 30 with secondary Sjögren's syndrome. The differences between both primary and secondary Sjögren's syndrome, in comparison to healthy controls, were significant (p < 0.001). The sensitivity of the method was found 90% for primary Sjögren's syndrome and 80% for secondary Sjögren's syndrome. According to our data, tear mucus ferning test is a simple, sensitive and specific test to estimate Keratoconjunctivitis sicca in patients with Sjögren's syndrome.