The aim of the study was to explore the process of the transition from junior to senior sports in Swedish athletes. Previous studies showed that the junior-to-senior transition (a) is initiated by a set of demands relevant to athletic and non-athletic development, (b) lasts for about two years, (c) known for a high dropout rate and often described by athletes as the most difficult within-career transition (e.g., Bruner et al., 2008; Stambulova, 2009; Vanden Auweele et al., 2004). This quantitative longitudinal study included five measurements that were conducted every six months, and altogether covered two-and-a-half-years with two measurements of the transition variables and one measurement of related personal variables each year. The following package of four instruments was used: the Transition Monitoring Survey (Stambulova, Franck, & Weibull, 2012), the Athletic Identity Measurement Scale (Brewer, Van Raalte, & Linder, 1993), the Task and Ego Orientation in Sport Questionnaire (Duda, 1989) and the Physical Self-Perception Profile – Revised (Lindwall, Hagger, & Asci, 2007). In the first measurement 101 club-based Swedish athletes (74 males and 27 females) of 15 -20 years old took part. The dynamics of participants was characterized by an increasing dropout rate from each measurement to the next, and as a result only 37 participants were left to the time of the final (fifth) measurement. Overall dynamics of transitional variables throughout the five measurements was characterized by an increase in motivation and perceived quality of adjustment on the senior athletic level from the first to the third measurement followed by a decrease in these variables across the last two measurements. Decrease in the athletes’ perceived degree of adjustment was especially relevant to their adjustment to senior competitions and to combining sport and studies. Perceived importance of sport (especially of competitions) decreased progressively from the first to the fifth measurement. The other transitional variables (e.g., perceived demands, resources, coping strategies, stress level, need in support) were characterized by various types of dynamics. Meanwhile athletes’ athletic identity and overall satisfaction with their sport and life were rather high and stable across all the five measurements. The next step in the data treatment will be based on the Multilevel Modeling and the Latent Growth Curve Analysis to identify successful and less successful transitional pathways with relevant dynamics and patterns of the transitional and personal variables.