Promoting sustainable physical activity (PA) behavior change is challenging, and a number of theoretical models have been developed and applied to this problem. Pender's health promotion model (HPM) is a relatively new model that is based on Bandura's social cognitive theory but includes the additional construct of competing demands, which are viewed as alternative behaviors (e.g., watching television) that have powerful reinforcing properties. This study evaluates the HPM as a means to predict PA in a sample of Iranian adolescent boys. Participants were 515 boys from 100 junior high and high schools in Sanandaj, Iran. Participants' mean age was 14.33 years (SD = 1.6, range = 12-17). Participants completed questions assessing social cognitive variables, and structural equation modeling was used to fit the data to the HPM. The HPM accounted for 37% of the variance in PA but did not represent a good data fit ([chi][superscript 2] = 913.85, df = 473, p less than 0.001). There were significant pathways between PA and self-efficacy ([beta] = 0.25, p less than 0.001), enjoyment ([beta] = 0.22, p less than 0.01), and PA modeling ([beta] = -0.13, p less than 0.05). A revised model that included the indirect effects of competing demands explained 34% of the variance in PA and represented a good data fit ([chi][superscript 2] = 9.12, df = 4, p = 0.058). In the revised model, self-efficacy, commitment to planning, and enjoyment were associated with PA. According to the HPM, competing demands influence PA. In the study sample, competing demands were not related to PA but were inversely associated with commitment to planning. (Contains 2 tables and 2 figures.)