An elderly woman in her late 80s was living with her husband and her son in his mid-60s. The woman had urinary incontinence. Her son took her to the bathroom, showered her in the bathtub with her clothes on, and then left her inside the bathtub. Five days after on a hot summer day, the woman was found dead inside the waterless bathtub. A forensic autopsy was performed 2 days after the discovery of her death on suspicion of the son’s negligence as a guardian. The woman was underweight, and the low BMI value in addition to the lack of sufficient subcutaneous fat and musculature, had an effect on the woman’s vulnerability to hypothermia. Furthermore, the woman’s clothes were drenched. We believe that these factors contributed greatly to her susceptibility to hypothermia. Examining all of the environmental factors and autopsy results, the cause of death was concluded to be hypothermia.