This paper provides a unique comparison between French and Portuguese local governments with respect to the nature and determinants of budget forecast errors. It starts by documenting and comparing their statistical properties. The results point at biased and inefficient budget forecasts, which seem to have been more cautious in French departments than in Portuguese municipalities. Second, we examine the political, institutional and economic determinants of forecast biases. Overall, we find that they are essentially driven by electoral motivations and by institutional differences across the two countries. In particular, opportunistic forecasting is more prevalent where governments enjoy greater margin of maneuver, and there is evidence of conservatism in French departments where fiscal autonomy is greater.