In this paper, we describe a highly efficient automatic test pattern generator for stuck-open (SOP) faults, calledSOPRANO, in CMOS combinational circuits. The key idea of SOPRANO is to convert a CMOS circuit into anequivalent gate level circuit and SOP faults into the equivalent stuck-at faults. Then SOPRANO derives testpatterns for SOP faults using a gate level test pattern generator. Several techniques to reduce the test set sizeare introduced in SOPRANO. Experimental results performed on eight benchmark circuits show that SOPRANOachieves high SOP fault coverage and short processing time.