We have reexamined the existing ground-based measurements of Io's thermal emission at infrared wavelengths of 8.4, 10.6 and 21 mm. Present in these data is the signature of 'hot spots,' presumably similar to the hot spots seen by the IRIS experiment on Voyager. It is possible to extract from these data the total amount of power radiated. Since the hot spots are believed to be a result of deep-seated activity in Io and since the remainder of Io's surface is an extraordinarily poor thermal conductor, the power radiated by the hot spots is essentially the total heat flow. Our analysis yields a heat flow of 2¿1 W m-2. This value is tremendously large in comparison to the average heat flow of the Earth (0.06 W m-2) and the moon (0.02 W m-2), but it is characteristic of active geothermal areas on the Earth. A heat flow this large requires that the interior of Io be at least partially molten on a global scale.