Ferromagnetism in 3d transition metal-atom-doped ZnO was investigated by ab initio electronic structure calculations based on the generalized gradient approximation (GGA). We also performed GGA + U calculations to further refine our results. We found that Mn-doped ZnO has the largest magnetic moment and Sc-doped ZnO is nonmagnetic. Also, MnCo- or Cu-doped ZnO are ferromagnetic semiconductors. For the other 3d transition metal atoms the doped ZnO are metals. The appearance of ferromagnetism in these systems suggests the possibility for fabrication of a transparent ferromagnet which may have a great impact on industrial applications in magneto-optical devices. (C) 2004 Published by Elsevier B.V.