We have developed a novel batch-type technology for making three-dimensional (3-D) millimeter-sized transformers for ultrasmall low-power (0.1 to 1 Watt) applications. The technology uses the 3-D micropatterning of ferrite wafers by powder blasting to form the magnetic cores of the inductive devices, and combines these cores with electrical windings made by flex-foil printed circuit board technology. Microfabrication and assembly of the parts can be done in a batch process on a wafer/foil level, opening the way to further size reduction of the components. We have measured the inductive and resistive properties of our devices as a function of frequency and device geometry. The results clearly show the high potential of our technology for power applications in which small-size is important.