We introduce an aggregate demand externality into the Mortensen-Pissarides model of equilibrium unemployment. Because firms care about the demand for their products, an increase in unemployment lowers the incentives to post vacancies which further increases unemployment. This positive feedback creates a coordination problem among firms and leads to multiple equilibria. We show, however, that the multiplicity disappears when enough heterogeneity is introduced in the model. In this case, the unique equilibrium still exhibits interesting dynamic properties. In particular, the importance of the aggregate demand channel grows with the size and duration of shocks, and multiple stationary points in the dynamics of unemployment can exist. We calibrate the model to the U.S. economy and show that the mechanism generates additional volatility and persistence in labor market variables, in line with the data. In particular, the model can generate deep, long-lasting unemployment crises.