OM-85 BV, an immunostimulant made from bacterial extracts, has been shown to reduce the risk of hospitalization for acute exacerbation in patients with chronic obstructive pulmonary disease, as well as to reduce the length of stay for all hospitalizations. In conjunction with a placebo controlled, randomized clinical trial, a cost effectiveness analysis was carried out to assess the economic impact of using OM-85 BV. In the analysis, effectiveness was defined as the difference in the number of severe acute exacerbations, assessed by the number of hospitalizations for a respiratory problem, between the placebo and OM-85 BV-treated groups. The median cost to prevent one day of hospitalization for a respiratory condition was CDN$45, with a 95% CI of CDN$18 to CDN$210. Bootstrap of the study population and sensitivity analyses showed that the results were robust and not likely due to random fluctuation; 98.8% of the cost effectiveness and 96.8% of the cost-benefit ratios favoured the use of OM-85 BV. Indirect costs, defined as a need for help, were reduced by 36% in the group treated with OM-85 BV: 779 h of help compared with 1212 h in the placebo group. This trend, while not significant, is consistent with other results and suggests a decrease in the severity of exacerbations in the OM-85 BV-treated group. Given the high prevalence of chronic obstructive pulmonary disease worldwide and the high cost of acute exacerbations, immunostimulants may become a key element in the improved control of this condition.