Friedel-Crafts acylation of 4-hydroxy-1-phenyl[1,8]naphthyridin-2(1H)-one (I) with benzoyl chloride (II) using AlCl3 in hot nitrobenzene produced the 3-benzoyl naphthyridinone (III). This was cyclized to the title pyrazolonaphthyridinone by treatment with hydrazine in refluxing HOAc.