The purpose of this article is to study whether moral stress is a phenomenon relevant to teaching practice and which may make a significant contribution to understanding why teachers repeatedly reported feeling burdened by work. Moral stress can be caused by acting in conflict with one's own conscience, e.g. when one knows the right thing to do but institutional constraints make it difficult to act in a way that is consistent with one's morals. The method used in this study is critical incident technique focusing ethical dilemmas in teaching. The findings add a phenomenon to previous research of moral stress in other professions; moral stress can be caused not only by external regulations, but also by internal moral imperatives in conflict with one another.