In the wing imaginal disc, the decapentaplegic (dpp) gene is expressed in a stripe of anterior cells near the anterior-posterior compartment boundary, and it is required solely in these cells for the entire disc to develop. In some viable segment polarity mutants, alterations in dpp expression have been demonstrated that correlate with changes in wing morphology. To test the hypothesis that the abnormal patterns of dpp expression are responsible directly for the mutant phenotypes, we have expressed dpp in ectopic places in wing imaginal discs, and we have found that dpp is able to cause overgrowth and pattern duplications in both anterior and posterior compartments of the wing disc. The alterations of the anterior compartment are strikingly similar to those observed in some viable segment polarity mutants. Thus, ectopic dpp alone can account for the phenotype of these mutants. We also show that ectopic expression of the segment polarity gene hedgehog (hh) gives similar morphological changes and activates dpp expression in the anterior compartment. This strongly suggests that the organizating activity of hh is mediated by dpp. We propose that the expression of dpp near the anterior-posterior compartment boundary is directed by the interaction between patched and hh, and that dpp itself could act as a general organizer of the patterning in the wing imaginal disc.