Physiological effects of a dog-assisted, stress management and communication training program conducted in a prison were investigated by assessing salivary cortisol concentrations of prison inmates before and after the sessions. The program was conducted with groups of inmates with psychiatric and/or developmental disorders. In the program, male inmates interacted with trained pet dogs and their volunteer handlers. Inmates evaluated their mood states before and after each session by using a questionnaire. Moreover, handlers evaluated the quality of the interaction with inmates after each session. Results indicated that the inmates' cortisol values in most cases decreased following their participation in the sessions. The inmates who demonstrated stress reduction by decreasing cortisol values were diagnosed only psychiatric disorders, were aware of their mood improvement, and were evaluated by the handlers as having interacted well during the sessions. This indicated the validity of psychiatrists' diagnoses, inmates' self-evaluation, and handlers' evaluation of inmates. Also, inmates who experienced a medium mood without mood changes had decreased cortisol values. It is suggested that these results are useful for predicting and selecting inmates who are expected to obtain effects by participating in the program. Moreover, it is suggested that providing feedback to inmates about changes in their salivary cortisol levels could help them better understand their psychophysical state, which could result in more effective stress management.