Two studies involved structured interviews with 203 homeless people to examine the impact of sociodemographic variables, prior behavior, and the theory of planned behavior (TPB) variables, attitudes, subjective norms, perceived behavioral control, and intention, on uptake of outreach services. Study 1 was conducted in London, and Study 2 involved a comparable sample in New York. In line with previous research, the psychological variables accounted for more variance than the sociodemographic variables, and there was no evidence that the effects of TPB variables were moderated by sociodemographics. In London intentions and behavior were most affected by perceived control and subjective norms. In New York they were most affected by perceived control and attitude. These differences may be attributable to different institutional structures and opportunities for service uptake in the two cities. Sensitivity of the TPB to social context and implications for intervention strategies are discussed in light of these differences. (Authors)