Background: Cyclosporine is an immunosuppressive agent used to treat immune-mediated and inflammatory diseases in dogs. We have developed a pharmacodynamic (PD) assay that measures interleukin-2 (IL-2) produced by activated T cells to measure the immunosuppressive effects of cyclosporine. Hypothesis/objectives: Our retrospective study extracted data from samples submitted to our laboratory to obtain descriptive statistics, to determine whether assay results predicted treatment effectiveness, and to determine whether cyclosporine formulation or breed affected PD responses. Animals: 1,110 samples were analyzed over 4 years. Methods: Extracted data was analyzed to determine whether there was a relationship between assay results and clinical control, and whether either formulation or breed affected results. Results: We found no relationship between assay results and control of signs, and found that breed did not affect results. At comparable doses, proprietary modified cyclosporine was more immunosuppressive than proprietary non-modified cyclosporine, and both proprietary and generic modified formulations had similar efficacy.