This study examined the relationships between American and South Korean students' achievement and their time spent in out-of-school activities. Analyzing nationally representative data for 8,912 U.S. and 5,309 Korean middle school students from the Trends in International Mathematics and Science Study (TIMSS) 2003, we found differences in students' time-spending patterns for out-of-school activities and associations between out-of school activities and academic achievement in the two countries. Although watching TV and playing with friends are the most common activities in both countries, Korean students spent more than 30% of their out-of-school hours using the Internet and playing PC games, whereas American students spent about 27% of their out-of-school hours playing sports and working at home or a paid job. Regarding the associations between out-of-school activities and achievement, playing sports was a positive predictor of achievement in the U.S. but a negative predictor in Korea, and doing homework was a negative predictor in the U.S. but a positive predictor in Korea. It was a positive predictor for high-achieving students in the U.S., but it was a negative predictor for low-achieving students in the U.S. By contrast, reading books was a common positive predictor of achievement in both nations. In addition, the relationships between achievement and the type of out-of-school activities differed by gender and achievement levels. These differences are greater among the lower achieving groups of each nation than the high achievers. Even though there are some limitations to the present study, these results may have important implications for parents and educators. (Contains 5 tables and 2 figures.)