OBJECTIVES: This study estimated the incremental costs of using olopatadine (Opatanol) compared to branded cromoglycate (Opticrom) and generic cromoglycate in the treatment of season allergic conjunctivitis (SAC) in the UK. METHODS: A literature-based decision model was constructed depicting the management of SAC sufferers >= 4 years of age over four months (a typical allergy season). The model considers the decision by a GP to initially treat a patient with olopatadine (1 drop in affected eyes twice daily), branded and generic cromoglycate (1 or 2 drops in affected eyes 4 times daily). Whilst published studies demostrate olopatadine's greater symptom reduction compared to cromoglycate, there was no evidence of any significant differences between the two treatments in terms of days of treatment and overall probability of being successfully treated. Therefore, for the purposes of this analysis both drugs were assumed to be equally effective. Consequently, a cost-minimisation analysis was performed to identify the least costly alternative from the perspective of the UK's National Health Service (NHS). RESULTS: Starting treatment with olopatadine is expected to lead to a healthcare cost of GBP 92 (95%CI:46-150)over four months compared to GBP 109 (95% CI:65-166) with branded cromoglycate and GBP 95 (95% CI: 51-152)with generic cromoglycate. Consequently, use of olopatadine instead of branded or generic cromoglycate is expected to lead to a 16% and 3% reduction in health care costs respectively over four months of treatment. This cost-difference is primarily due to fewer GP visits among olopatadine-treated patients. CONCLUSION: Use of olopatadine instead of branded or generic cromoglycate affords an economic benefit to the NHS. Hence, within the limitations of our model, olopatadine is the preferred first-line treatment for use in SAC sufferers, since it is expected to release healthcare resources for alternative use and may offer better symptom reduction to patients.