OBJECTIVE: To investigate fatty acid composition in the diet, plasma phospholipids, and adipose tissue in a cohort of patients with ankylosing spondylitis (AS), and to determine their correlations to disease activity and blood lipids in a cross-sectional study. METHODS: Diet was assessed using a food frequency questionnaire in 66 patients with AS. Polyunsaturated fatty acids in plasma phospholipids and gluteal adipose tissue were measured using gas chromatography. Disease status was quantified using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), erythrocyte sedimentation rate (ESR), high sensitivity C-reactive protein, and proinflammatory cytokines. RESULTS: Diet did not correlate with disease activity assessed by the BASDAI, but there were negative correlations between the dietary intake of long-chain omega-3 fatty acids and ESR (rs = -0.27, p < 0.05). The plasma phospholipid content of arachidonic acid correlated significantly with the BASDAI score (rs = 0.39, p < 0.01). There were correlations between the intake of long-chain omega-3 fatty acids and high-density lipoproteins and serum triglycerides (rs = 0.26 and rs = -0.25, respectively, p < 0.05). CONCLUSION: There was a positive correlation between levels of arachidonic acid in plasma phospholipids and disease activity assessed by BASDAI in patients with AS. A Western diet does not appear to influence this correlation, but seems to affect blood lipids involved in atherogenic processes.