OBJECTIVE To assess the alterations of the tear film lipid layer and tear functions in patients with seasonal allergic conjunctivitis (SAC) and to compare the results with healthy control subjects. METHODS Seventy-eight eyes of 39 consecutive patients diagnosed as SAC (mean age 32.6 years; 11 male, 28 female) as well as 20 eyes of 10 healthy control subjects (mean age 32.5 years; 6 male, 4 female) underwent slit-lamp examinations, tear film breakup time measurements (BUT), corneal fluorescein stain scoring, Schirmer test, and tear film lipid layer interferometry. The 2 groups were then compared for the examined parameters. RAST and serum IgE level evaluations were also carried out in the patients to confirm the diagnosis of allergy. RESULTS The mean BUT was 3.4 +/- 1.5 seconds in patients with SAC compared with the mean value of 12.4 +/- 2.4 seconds in the controls (P < 0.05). There were no significant differences in relation to Schirmer test values between the 2 groups; 78% of the patients with SAC had grade 3 or above dry eye change in tear film lipid layer interferometry, whereas none of the controls had an interferometry grade greater than 3 (grade 1-2 normal; grade 3-4 dry eye; grade 5 severe dry eye). Eyes with SAC had significantly higher tear film lipid layer thickness ranges compared with the control eyes (P < 0.05). CONCLUSIONS SAC was associated with advanced tear instability and thickening of the tear film lipid layer. Evaluation of the tear film lipid layer thickness might be useful in the assessment of the extent of dry eye disease and the treatment outcomes in patients with allergy.