Scientific successfulness of research trainees The aim of this study was to provide a detailed evaluation of the research trainees support system of the Ministry of Science, Education and Sports of the Republic of Croatia (MSES). Trainees whose support has terminated during 1999-2005 were included in this study. The data were obtained from the MSES human resources database (n=1,320). Additionally, trainees who were employed in 2006 were surveyed (n=2,093), as well as their mentors, in order to obtain additional information and mentor's feedback on their career development. The data were analysed with data mining and statistical methods. The results indicate that a third of research trainees managed to obtain a PhD degree during employment (32.9%), the most in natural sciences, and the least in biomedicine. A third of research trainees managed to get a permanent employment within the MSES support system after their fellowship has terminated (36.1%). Finally, a third of research trainees managed to publish at least one scientific article that is indexed in the Web of Science database. The results also support the view that mentor's input is an important factor in the research trainee’s career advancement. Based on these findings, this study suggests several ways in which the support system could be improved. These mainly include enhancement of the selection process, improvement of the research trainee’s career advancement scheme and creation of the information system that could be used to monitor research trainees and their career advancement.