This paper deals with millimeter-wave microstrip antenna arrays. The objectives were to develop and optimize a microstrip-to-thick-slot-line transition near 40 GHz, and to feed a printed array with this kind of transition. The influence of such a transition between the two layers, and its effect on the radiation properties (pattern symmetry, sidelobe level) have been checked. A study of the microstrip/thick-slot/microstrip transition is first presented. Then, a comparison is made between radiated fields of arrays fed directly and through slot transitions. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 51–53, 1999.