We present a new evaluation criterion for the induction of decision trees. We exploit a parameter-free Bayesian approach and propose an analytic formula for the evaluation of the posterior probability of a decision tree given the data. We thus transform the training problem into an optimization problem in the space of decision tree models, and search for the best tree, which is the maximum a posteriori (MAP) one. The optimization is performed using top-down heuristics with pre-pruning and post-pruning processes. Extensive experiments on 30 UCI datasets and on the 5 WCCI 2006 performance prediction challenge datasets show that our method obtains predictive performance similar to that of alternative state-of-the-art methods, with far simpler trees.