Two suicidal cases associated with ingestion of diphenhydramine (DPH) were reported. Case 1 is a typical DPH overdose case of a young man with the blood DPH concentration of 12.2 microg/ mL. Case 2 is a double suicide of a man and a woman. They ingested DPH and fell asleep in a vehicle which had a cooking clay charcoal stove. Their blood DPH concentrations were 0.4 and 0.7 microg/mL, which were high enough to make them sleep. Their cause of death, however, was carbon monoxide poisoning with blood CO-Hb concentration of 14 and 19%. DPH is a low toxic agent and is available as an OTC drug in Japan. Similar fatal cases can be expected to happen in Japan.