Missense mutations in the PTPN11 gene, which encodes the protein tyrosine phosphatase SHP-2, cause clinically similar but distinctive disorders, LEOPARD (LS) and Noonan (NS) syndromes. The LS is an autosomal dominant disorder with pleomorphic developmental abnormalities including lentigines, cardiac defects, short stature and deafness. Biochemical analyses indicated that LS alleles engender loss-of-function (LOF) effects, while NS mutations result in gain-of-function (GOF). These biochemical findings lead to an enigma that how PTPN11 mutations with opposite effects on function result in disorders that are so similar. To study the developmental effects of the commonest LS PTPN11 alleles (Y279C and T468M), we generated LS transgenic fruitflies using corkscrew (csw), the Drosophila orthologue of PTPN11. Ubiquitous expression of the LS csw mutant alleles resulted in ectopic wing veins and, for the Y279C allele, rough eyes with increased R7 photoreceptor numbers. These were GOF phenotypes mediated by increased RAS/MAPK signaling and requiring the LS mutant's residual phosphatase activity. Our findings provide the first evidence that LS mutant alleles have GOF developmental effects despite reduced phosphatase activity, providing a rationale for how PTPN11 mutations with GOF and LOF produce similar but distinctive syndromes.