BACKGROUND: Two extracorporeal photopheresis (ECP) instruments, the CELLEX and the UVARXTS are currently being used “off-label” in the US for treatment of graft versus host disease (GVHD). Our study compared the performance of the two instruments in the setting of acute and chronic GVHD. STUDY DESIGN AND METHODS: We retrospectively analyzed the outcomes of patients with steroid refractory or steroid resistant GVHD undergoing ECP at Barnes Jewish Hospital. Multivariate logistic regression was used to evaluate the comparative efficacy of the two instruments with respect to steroid dose reduction (≥50% from baseline) and clinical improvement in GVHD. Chi-square/Fisher exact tests were used to compare the incidence of adverse events, while multivariate Cox regression was employed to assess a potential difference in mortality between the two instrument treatment cohorts. RESULTS: After adjusting for potential confounders, there was no significant difference in the odds of steroid dose reduction (OR = 1.41, 95% confidence interval [CI]: 0.51-3.90, p = 0.50) or clinical improvement (OR 2.0, 95% CI: 0.63-6.41, p = 0.24) between the two instrument treatment cohorts. The frequency of adverse events (CELLEX 45.4%; UVAR XTS 40.5%, p = 0.55) was also comparable between the cohorts. There was no significant difference in mortality of either acute or chronic GVHD patients when treated by the CELLEX as compared to the UVAR-XTS (aHR 0.66, 95% CI: 0.35-1.25, p = 0.20). CONCLUSION: The efficacy and safety of the two ECP instruments, the CELLEX and the UVAR-XTS, are comparable for the treatment of acute and chronic GVHD.