Despite the fact that coaching has been established as an important tool within human resource development, research on its effectiveness is limited. Contrary to related research, the analysis of negative effects has been particularly neglected. For the first time, negative effects of coaching are systematically explored. Negative effects were determined within a qualitative preliminary study. Subsequently, their frequency, intensity, and length were rated for 123 coachings from the coaches’ perspective. Results indicate an average of two negative effects per coaching; whereas the majority of negative effects occurs with a rather low intensity. Additionally, the coaches were asked to rate causes of negative effects. Most of all, they chose factors associated to clients, but also factors such as the coaches approach to the psychological illnesses of clients or limited time resources. Moreover, a positive relationship between the number of topics considered and the frequency of negative effects was detected.