Positive fantasies allow people to mentally indulge in a desired future. Whereas previous research found that spontaneously generated positive fantasies about the future predict poor achievement, we examined the effect of experimentally induced positive fantasies about the future. The present four experiments identify low energy, measured by physiological and behavioral indicators, as a mechanism by which positive fantasies translate into poor achievement. Induced positive fantasies resulted in less energy than fantasies that questioned the desired future (Study 1), negative fantasies (Study 2), or neutral fantasies (Study 3). Additionally, positive fantasies yielded a larger decrease in energy when they pertained to a more rather than a less pressing need (Study 4). Results indicate that one reason positive fantasies predict poor achievement is because they do not generate energy to pursue the desired future.