Radon concentrations measured from July 1990 to January 1993 at Gyokusen-do limestone cave on Okinawa vary from 10 to 3000 Bq/m³ and show significant and consistent seasonal variations, higher in summer and lower in winter. Concentration variations are controlled mainly by the exchange of air between external and internal cave air. During winter inblowing external cold air with a large density exchanges with cave air and dilutes the radon. In the summer, when the external air is relatively warmer than the cave air, the cold cave air stagnates and radon accumulates.