OBJECTIVES: To evaluate the cost efficiency of preventive treatment with OM 85 in patients with AR, asthma and COPD compared to the same patients the previous year only receiving standard of care. METHODS: This multi-centre, clinical trial was conducted in Argentina in 2010. Eighty-four  patients with COPD, allergic rhinitis (AR) and asthma aged 16-65 years who had not received a bacterial lysate in 2009 were included. In 2010, the same patients  received OM-85 bacterial lysate capsule. Capsules were administered daily for 10 consecutive days per month for a duration of 3 consecutive months. RESULTS: the number of reinfections and exacerbations in the OM 85 arm was decreased from 85% to 45.5% (p< 0.05) and from 65.7% to 34.9%  (p<0.05) vs the previous year, respectivelyHospitalizations were 2% in the OM 85 group and 12% for the previous year. The average total cost per patient per month with AR caused by  reinfections and exacerbations was 448.9 ARS and  269.9 ARS in the OM 85 arm compared to 660.40 ARS and  574.40 ARS in the previous year. In patients with asthma total cost for reinfections and exacerbations was 487.9 ARS and 473.9 ARS in the OM 85 arm compared to 1’144 ARS and 970 ARS in the previous year. Reinfections and exacerbations total cost in COPD patients was 1356.5 ARS and 1217.5 ARS in the OM 85 patients but amounted to 1’708.2 ARS and 1599.6 ARS, respectively, in the previous year. CONCLUSIONS: Preventive treatment with OM 85 in patients with allergic rhinitis, asthma and COPD significantly reduces reinfections rates, exacerbations and hospitalizations compared to the previous year. Associated cost savings have been substantial and ranged from 15% to 60% vs previous year (no prophylaxis).