In Taiwan, a subtropical country without any history of heat waves, heat stroke has been considered a rare disease. However, after seeing several cases of the classic type of heat stroke at the end of the summer of 1998 (an unusual event) we began to review and collect cases of suspected heat stroke (hyperthermia (>40.6 degrees C) in the presence of altered mental status and anhidrosis) and tried to explore the possible cause of this unusual phenomenon. Through a emergency department (ED) chart review, case retraction from International Classification of Diseases (ICD) code, and ED conferences, six patients were found for the period from June to August (the hottest months in Taiwan) 1998. We found that the most common comorbid conditions were hypertension (4/6) and preexisting mental problems (3/6). All patients lived in the inner part of an urban area, were middle class, and were not socially isolated. Most of our patients felt unhealthy being exposed to the cold and avoided staying in air-conditioned rooms. Laboratory abnormalities and clinical presentations, except for a high fever and conscious change, seemed to be nonspecific. All cases occurred during two periods of sustained hotter-than-average weather and, to our surprise, we found that three episodes occurred around the day of the highest weather temperature (38.1 degrees C). However, the higher temperatures (around 30 degrees to 31 degrees C and 32 degrees to 33 degrees C) did not reach the criteria of a heat wave. Compared with the other study, our patients seemed to have initial worse outcomes. Because of special environment and social factors, classic heat stroke may occur occasionally in subtropic regions, without previous history of heat waves and where heat stroke rarely occurs, during periods of the persistently high temperatures. Prevention of heat stroke in an area with a low incidence includes early health organizations' issue of advisories or warnings through the media and reminding or teaching emergency physicians about heat stroke during sustained hot weather, especially when record temperatures are set.