The widely employed anti-diabetic drug pioglitazone (Actos) is shown to be a specific and reversible inhibitor of human monoamine oxidase B (MAO B). The crystal structure of the enzyme-inhibitor complex shows the R-enantiomer is bound with the thiazolidinedione ring near the flavin. The molecule occupies both substrate and entrance cavities of the active site establishing non-covalent interactions with the surrounding amino acids. These binding properties differentiate pioglitazone from the clinically used MAO inhibitors, which act through covalent inhibition mechanisms and do not exhibit a high degree of MAO A versus B selectivity. Rosiglitazone (Avandia) and troglitazone, other members of the glitazone class, are less selective in that they are weaker inhibitors of both MAO A and MAO B These results suggest that pioglitazone may have utility as a "re-purposed" neuro-protectant drug in retarding the progression of disease in Parkinson's patients. They also provide new insights for the development of reversible isoenzyme-specific MAO inhibitors.