Home prices in the United States often exhibit little (and sometimes even negative) correlation across different regions. This reflects segmentation in the national housing market and also provides an apparent opportunity for investors to diversify their exposure to regional downturns by creating residential mortgage-backed securities (RMBSs) out of geographically dispersed home loans. Unfortunately, in a crisis, correlations may rise, and the benefits from geographical diversification may disappear just when investors most desire them. Using a flexible generalized autoregressive conditional heteroscedasticity (GARCH) technique, I find that regional correlations indeed rose dramatically during the latest downturn, in some cases to unprecedented levels. Moreover, this increase in co-movement was clearly financial contagion, and not merely interdependence. Investors in mortgage-backed and other housing securities should thus not rely on house price correlations calculated during “normal” times.