The purposes of the present research were to clarify the structure of relationships between senior and junior members (senpai and kohai) of junior high and high school clubs, and to clarify the extent of differences in the senior-junior relationships based on factors such as the type of club activity, students’ academic year, and gender.  The research also aimed to examine the extent to which senior-junior relationships could be predicted on the basis of a description of the clubs’ activities and characteristics of those activities.  Junior high (n=344 ; 166 boys, 178 girls) and high school (n=367 ; 140 boys, 227 girls) students nationwide completed questionnaires.  The results suggested that the first-year students reported that the senior-junior relationship was the most important one during both their junior high and high school years.  The junior high students also showed a trend for the female students taking the senior-junior relationship more seriously than the male students did.  These results suggest that the senior-junior relationships is more established in the activities of clubs that engage in a high level of competition, such as in sports competitions and contests, and also in sports clubs.  It was possible to predict the policy and personality of the clubs’ activities with a high degree of certainty, based on various aspects of the senior-junior relationships in those clubs.