The goal of this research was to explore differences between lay and professional explanatory models both within and between two countries. We test which effect is stronger, country of residence or professional/lay status, in determining similarities and differences of explanatory models of AIDS. Interviews conducted in Guadalajara, Jalisco (Mexico) and the Edinburg-McAllen area of south Texas (United States) elicited explanatory models of AIDS. Two pairs of samples were interviewed: a physician and community sample in Mexico and a physician and community sample in the United States. Comparisons of the explanatory models indicated that there was a shared core model of AIDS across all four samples, but that physicians’ models were more similar to those of lay people in their own communities than either was to samples across the border.