Gain-of-function mutations in nicotinamide adenine dinucleotide phosphate-dependent isocitrate dehydrogenase (IDH)1 and IDH2 frequently arise in human leukemias and other cancers and produce high levels of D-2-hydroxyglutarate (D-2HG). We expressed the R195H mutant of Drosophila Idh (CG7176), which is equivalent to the human cancer-associated IDH1-R132H mutant, in fly tissues using the UAS-Gal4 binary expression system. Idh-R195H caused a >25-fold elevation of D-2HG when expressed ubiquitously in flies. Expression of mutant Idh in larval blood cells (hemocytes) resulted in higher numbers of circulating blood cells. Mutant Idh expression in fly neurons resulted in neurologic and wing-expansion defects, and these phenotypes were rescued by genetic modulation of superoxide dismutase 2, p53, and apoptotic caspase cascade mediators. Idh-R163Q, which is homologous to the common leukemia-associated IDH2-R140Q mutant, resulted in moderately elevated D-2HG and milder phenotypes. We identified the fly homolog of D-2-hydroxyglutaric acid dehydrogenase (CG3835), which metabolizes D-2HG, and showed that coexpression of this enzyme with mutant Idh abolishes mutant Idh-associated phenotypes. These results provide a flexible model system to interrogate a cancer-related genetic and metabolic pathway and offer insights into the impact of IDH mutation and D-2HG on metazoan tissues.