BACKGROUND: Obesity is prevalent in individuals with serious mental illness (SMI). OBJECTIVE: The purpose of this study is to examine the feasibility of a telephone-delivered physical activity (PA) intervention for outpatients with serious mental illness to maintain PA and to explore the preliminary efficacy of the intervention on health status. DESIGN: This study used a randomized experimental design. The treatment group received pedometers and eight weekly phone calls; the control group received written information regarding PA. Descriptive statistics were used to analyze data collected at baseline and 8 weeks. RESULTS: Twenty-two subjects with SMI (mean age = 44.09 ± 7.6 years; 54.5% were male) were recruited and 16 subjects completed the study in 8 weeks. PA (z = −2.37, p = .02) increased in the treatment group (n = 8) whereas the control group (n = 8) maintained baseline PA level (z = −1.61, p = .11). Health outcomes were not changed (ps > .05). CONCLUSION: Telephone-delivered intervention is feasible and has the potential to improve PA in individuals with SMI.