Thermotherapy has been applied to various joint diseases and injuries, but its direct effects on articular cartilage have remained unclear. The present study examined the effects on cell viability and metabolism by using the chondrocyte-like cell line HCS-2/8. The temperatures and durations of heat stimulation were 39 degrees, 41 degrees, 43 degrees, and 45 degrees C for 15 or 30 min. After heat stimulation of 41 degrees C or lower for 15 or 30 min, cell viability increased and proteoglycan metabolism was accelerated, whereas after stimulation at 43 degrees C or higher for 30 min the viability and metabolism decreased. These results indicate that appropriate heat stimulation positively affects cell viability and the proteoglycan metabolism of articular cartilage, whereas too much heat stimulation produces negative effects. Clinical efficacy is therefore determined by the overall thermal dose. When the appropriate combination of temperature and duration is found, thermotherapy for diseases and injury of articular cartilage can be highly useful in clinical practice.