Epidemiologic studies of mobile phone users usually rely on self reporting or billing records to assess exposure. In this study RF output data were collected using software-modified phones that recorded power control settings, coupled with a mobile system that recorded and analyzed RF fields measured in a phantom head (SYNEHA) in a vehicle over urban, suburban, and rural routes. The data were analyzed using analysis of variance methods. The findings suggest that phone technology (older analog produced the highest RF levels), and to a lesser extent, degree of urbanization (higher RF power output in rural areas), are the two stronger influences on RF power output. Software-modified phones should be useful for improving epidemiologic exposure assessment.