The objective of this study was to clarify the influences of sports activity in junior high-school athletes on bone strength and fractures. The subjects were 57 male junior high-school athletes aged from 13 to 15 years. They trained 120min, 6 times /wk. As an index of the bone strength, we measured the speed of sound of tibia (t-SOS) using Sound Scan 2000 Compact (Myriad Ultrasound System Ltd., Israel). The fracture rate was compared between the junior high-school athlete and control groups. The following results were obtained.<br>(1) In the junior high-school athlete group, their t-SOS was significantly lower than control group.<br>(2) The value of t-SOS demonstrated a negative correlation with the length of tibia/height ratio in the junior high-school athlete and control groups.<br>(3) The value of t-SOS demonstrated a positive correlation with calcium intake from dairy products in the junior high-school athletes.<br>(4) The junior high-school athletes showed a very high fracture rate. Particularly, the highest fracture rate (26.7%) was seen in the 13-year-old in the junior high-school athletes.<br>These data will be useful for designing appropriate exercise programs to support the growth and bone mineral acquisition in the junior high-school athletes.