BACKGROUND: Although several dietary factors have been reported to alleviate or aggravate the symptoms of irritable bowel syndrome (IBS), no information is available linking habitual dietary patterns to irritable bowel syndrome. OBJECTIVE: This study was undertaken to assess the association between dietary patterns and the risk of IBS among Iranian adults. METHODS: In this cross-sectional study, data on 3846 Iranian adults working in 50 different health centers were examined. Dietary intake of study participants was assessed using a 106-item self-administered Dish-based Semi-quantitative Food Frequency Questionnaire (DS-FFQ) which was designed and validated specifically for Iranian adults. To identify major dietary patterns based on the 39 food groups, we used principal component analysis. A modified Persian version of the Rome III questionnaire was used for assessment of IBS. RESULTS: We identified four major dietary patterns: (i) 'fast food', (ii) 'traditional', (iii) 'lacto-vegetarian', and (iv) 'western' dietary pattern. After adjustment for potential confounders, we found that those in the highest quartile of 'fast food' dietary pattern were tended to have higher risk of IBS than those in the lowest quartile (OR = 1.32; 95% CI: 0.99, 1.75, p = 0.05). An inverse association was also found between 'lacto-vegetarian' dietary pattern and risk of IBS; such that even after adjustment for potential confounders, those in top quartile of this dietary pattern were 24% less likely to have IBS (0.76; 0.59, 0.98; p = 0.02). No overall significant associations were observed between 'traditional' and 'western' dietary patterns and risk of IBS, either before or after adjustment for covariates. CONCLUSION: We found that 'lacto-vegetarian' dietary pattern was associated with reduced risk, while 'fast food' dietary pattern was associated with a greater risk of IBS in Iranian adults.