A patient committed suicide with hydrogen sulfide (H(2)S) by combining two commercial products. The patient was given hydroxocobalamin as an antidote in addition to treatment with cardiopulmonary resuscitation, but died approximately 42 min after his arrival at the hospital. The patient's cause of death was attributed to acute hydrogen sulfide poisoning. Serum concentrations of sulfide before and after administration of hydroxocobalamin were 0.22 and 0.11 μg/mL, respectively; serum concentrations of thiosulfate before and after hydroxocobalamin administration were 0.34 and 0.04 μmol/mL, respectively. Hydroxocobalamin is believed to form a complex with H(2)S in detoxification pathways of H(2)S. Although H(2)S is rapidly metabolized and excreted, the decreased sulfide concentration may be also associated with this complex formation. The decreased sulfide concentration suggests that hydroxocobalamin therapy may be effective for acute H(2)S poisoning. The decreased thiosulfate concentration seems to be associated with formation of a thiosulfate/hydroxocobalamin complex, because hydroxocobalamin can form a complex with thiosulfate. The thiosulfate concentration decreased to a greater extent than did sulfide, suggesting that hydroxocobalamin has a higher affinity for thiosulfate than for H(2)S. Therefore, prompt administration of hydroxocobalamin after H(2)S exposure may be effective for H(2)S poisoning.