This case study includes three pig production systems belonging to two companies in Spain. Mortality, percent culls, average daily gain (ADG), and feed efficiency in Production Systems One and Two were incorporated into a database program and analyzed using statistical process control (SPC) techniques to assess changes in performance before and after phytosterols, natural substances that act as immunomodulators, were added to the feed. Inmunicin Maymo (Maymo Laboratories SA, Barcelona, Spain), a commercial phytosterol product, was administered in feed during the nursery and finishing periods, from 4 weeks before until 4 weeks after the predicted date of an outbreak of porcine respiratory disease complex (PRDC). In Production System Three, data obtained for batches treated or not treated with Inmunicin Maymo were compared using a one-way ANOVA, with the level of significance set at .05. In all three production systems, finisher mortality and percent culls were lower and production parameters were best when the immunomodulator was applied. Differences were statistically significant for all parameters evaluated, except feed conversion ratio, when assessed using SPC criteria in Systems One and Two and one-way ANOVA in System Three. Phytosterols may be useful to control endemic PRDC under field conditions.