This research investigates the effects of Portland’s urban growth boundary (UGB) on urban development patterns and mobility. Three different methods are adopted for evaluating Portland’s UGB: intermetropolitan comparisons; comparisons inside and outside the UGB; and, statistical analyses utilising regression models. Intermetropolitan comparisons do not support the conclusion that Portland’s UGB has been effective in slowing down suburbanisation, enhancing infill development and reducing auto use. A significant level of spillover from the counties in Oregon to Clark County of Washington took place during the 1990s, indicating that the UGB diverted population growth into Clark County. Results from the statistical analyses also support the above findings. The UGB dummy variable was not significant during the 1980s and 1990s, indicating that the UGB had little impact on the location of new housing construction during the 1980s and 1990s. Unlike the UGB, the Clark County dummy variable is significant for both models, supporting the spillover effects of the UGB.