Hairy-wing (Hw) mutations are caused by modifications of the achaete-scute complex (AS-C) which promote development of extra sensory organs on the cuticle of Drosophila melanogaster. We show that the extreme Hw49c allele contains an inversion with a breakpoint within the AS-C, while the weak Hw685 allele is associated with a terminal deletion of the X chromosome which removes the achaete region of the AS-C. In both cases, foreign DNA in contact with the breakpoints presumably enhances expression of AS-C genes. Overexpression of achaete (T5) or scute alpha (T4) genes was previously found in Hw mutants associated with insertions of transposable elements (Hw1, HwBS and HwUa, Campuzano et al., 1986). In situ hybridizations to Hw49c and Hw1 larval sections show that the overexpression causes an abnormally generalized distribution of T4 and/or T5 transcripts in imaginal discs. Such distribution correlates with development of extra sensory organs in ectopic positions. We also show that in Hw685 a moderate overexpression of the T4 gene largely replaces the absence of the T5 gene in the development of the notum chaetae pattern. We propose that overexpression of T4/T5 genes in normal or ectopic positions is at the basis of the Hw effect.