The author first estimates trivariate vector autoregressions with gross worker flows and vacancies and identifies an aggregate shock by imposing only minimal sign restrictions on the responses of worker flows and employment growth and no restrictions on the response of vacancies. The data strongly suggest a hump-shaped and persistent response of vacancies. The calibrated model, on the other hand, predicts that vacancies respond to aggregate shocks with no delay and are not persistent even though an aggregate productivity shock is assumed to be highly persistent. These problems in vacancy behavior also cause gross flow series to exhibit counterfactual cyclical properties.