Scalar flat instantons (complete scalar flat metrics) show up regularly in the study of extremal Kahler metrics, and understanding such instantons is crucial in several approaches to realizing extremal metrics on Kahler 4- manifolds. Unfortunately there is no classifcation of such instantons, and itseems out of reach in the general case. In the toric case (where two holomorphic killing fields exist), such a classifcation has been studied by several authors, and is within reach. Several families of such metrics are already known, including explicit examples of the Kronheimer A-series ALE instantons (from his ADE classifcation). In this talk, we install a missing piece, namely a uniqueness theorem for such instantons. 1