In an agile business environment, teamwork quality is underlining key element of successful projects and overall team performance. In the Scrum framework, self-managing team is a central element of success. Self-managing teams introduced a new concept of teams and emphasized the importance of individual member effort and the way it affects team performance. The focus of this paper is on teamwork quality in self-managing teams and the way it affects the overall team performance. Teamwork quality is defined by six subconstructs: communication, coordination, balance of member contributions, mutual support, effort, and cohesion, while team performance is defined with effectiveness and efficiency. A survey of 260 respondents from the IT sector in Bosnia and Herzegovina was conducted and in both tested models, our results have confirmed that the highest effect on team effectiveness comes from mutual support, with slightly different – but still significant, impact of team cohesion and effort.