OBJECTIVE: The aim of this study was to compare the costs and clinical consequences of olopatadine, a new topical chemical entity with a dual mechanism of action (anti-histamine and mast cell stabilizer) to those of topical cromolyn sodium (CS) in the treatment of seasonal allergic conjunctivitis in Belgium, France, Germany, Netherlands, Norway, Portugal and Sweden. METHODS: A randomized, controlled, double-masked, multi-country clinical trial compared the efficacy and safety of olopatidine 0.1% bid and cromolyn sodium 2% qid. An economic comparison of first line and first-line failure treatment with olopatadine versus CS was modeled using clinical trial results and a standard cost approach. A societal perspective was adopted. Cost of failure was established from Pinto (2001). RESULTS: A total of 185 patients (91 olopatadine, 94 CS) presenting with SAC were treated over 42 days. At day 42, olopatadine-treated patients had lower itching (P <0.05) and redness (P <0.05) scores. The first-line, treatment-failure rate was 12.5% less (P <0.02) in olopatadine-treated patients. Olopatadine patients had a 1.6 greater chance (P<0.0001) of having a day without symptoms, from day 1 to day 42. Olopatadine was as safe as CS and well tolerated. According to Pinto, cost of failure varied across countries from 48 € to 72 €. Savings per episode due to avoiding failures with olopatadine were 7.00 € in Belgium, 8.68 € in France, 8.66 € in Germany, 6.12 € in NL, 6.02 € in Norway, 8.43 € in Portugal and 8.96 € in Sweden. Sensitivity analyses were conducted which confirmed the robustness of our findings. CONCLUSION: Based on results of a randomized clinical trial, and resources and costs associated with failure estimated from the literature, our model found that olopatadine is a cost-saving alternative to CS that offers more clinical benefits to patients. Results were consistent over all study countries.