The agranulocytosis associated with pyrazolone drugs have been considered using theoretical calculations at the DFT/B3LYP level of theory, with the 6-31G* basis set, in attention to the possible hindrance of neutrophil burst and scavenging activity of reactive oxygen species for chemistry mechanism elucidation. The pyrazolone drug, such as antipyrine, isopropylantipyrine, aminopyrine, and dypirone was used in this study. The calculations of HOMO and IP showed the oxidation in the initial electron of pyrazolone is related with more active compounds. HOMO and IP values showed that aminopyrine and dypirone are more efficient and toxic, while isopropylantipyrine had little efficiency and antipyrine had no biological or toxicological effects. The spin densities showed aminopyrine and dypirone were highly resonance structure at the pyrazolone ring than isopropylantipyrine and antipyrine.HOMO, LUMO, and MEPs showed differences between more toxic and less toxic compounds on agranulocytosis potential. Our results explain structural differences of pyrazolone derivatives on agranulocytosis.