OBJECTIVE To determine whether a synthetic feline facial pheromone product would decrease stress scores and upper respiratory tract infection (URI) incidence in shelter-housed cats. DESIGN Randomized controlled clinical trial. ANIMALS 336 stray, feral, owner-relinquished, or legally impounded cats at 2 animal shelters in northern California. PROCEDURES 5 cat holding rooms (3 at shelter A and 2 at shelter B) were used. A diffuser containing either synthetic pheromone or placebo was randomly assigned to each room, and cats were exposed for a 21-day period. Data collected on each cat included signalment, daily stress scores, and daily URI incidence. After 21 days, diffusers were removed for a 7-day washout period. The type of diffuser in each room was switched, and data were collected for another 21 days. Findings were statistically compared between exposure types and other groupings. RESULTS Cox proportional hazard analysis revealed no significant difference between exposure (pheromone or placebo) and URI incidence. Mixed-effects ordinal logistic regression revealed no significant relationship between exposure and daily stress scores. Three covariates had significant ORs: number of days in holding (OR, 0.80; 95% confidence interval [CI], 0.76 to 0.84), owner-relinquished versus stray (OR, 3.25; 95% CI, 1.18 to 8.94), and feral versus adult cat room at shelter A (OR, 11.10; 95% CI, 4.47 to 27.60). CONCLUSIONS AND CLINICAL RELEVANCE No evidence was found that the evaluated synthetic feline facial pheromone product had any effect on stress scores or URI incidence in shelter-housed cats. Therefore, other established methods for stress and URI reduction should be used in shelter settings.