The Internet has the potential for delivering innovative, interactive physical activity (PA) interventions to large numbers of people. This study was designed to test the efficacy of an Internet intervention that consisted of a Web site plus 12 weekly e-mail tip sheets, compared with a waiting list control group. The Internet intervention was theory based and emphasized clear, graphical presentation of PA information. Sixty-five (30 intervention and 35 control) sedentary adult employees of several large hospitals (9 men and 56 women) were randomly assigned to 1 of the 2 study arms. Of the 65 participants, 57 completed the 1-month follow-up, and 52 completed the 3-month follow-up. At both 1 and 3 months, those in the intervention group were significantly more likely to have progressed in stage of motivational readiness for PA than participants in the control group: 1 month, c2(1, N =52) =4.05, p <.05; 3 months, c2(1, N =52) =6.45, p <.01. We hypothesized that at 1 and 3 months, the intervention group would exhibit significant increases relative to the control group on the number of minutes of moderate activity. At the 1-month assessment, the intervention group did exhibit significant increases, relative to the control group in moderate minutes, F(1, 54) =5.79, p <.05; however, at the 3-month assessment this difference was no longer significant. In addition, secondary analyses were conducted to examine total number of minutes of walking reported. At 1 month, the intervention group did exhibit significant increases, relative to the control group, in walking minutes, F(1, 54) =12.1, p <.001. At the 3-month assessment, amount of time spent in walking activity continued to be significantly higher for the intervention group compared with the control group, F(1, 48) =5.2, p <.05. These findings show that a theoretically based PA Web site and weekly e-mail tip sheets can have a short-term impact on PA motivation and behavior both at 1 and 3 months. As Internet access increases, and as bandwidth and other technical attributes of this medium improve, Web site delivered health behavior interventions will become increasingly useful in public health promotion.