The present study examined the processes of supervisees’ non-disclosure in the supervision of psychotherapy. The purpose of the study was to determine the frequency of non-disclosure by supervisees, the content of non-disclosure and the reasons for it. The study involved 50 supervisees (42 women and 8 men), with different professional developmental levels and belonging to various psychotherapeutic schools. Study participants completed the Questionnaire of Significant Aspects of Supervision after each of two consecutive supervision sessions. Half of the sessions were conducted in individual and half of the sessions in group settings. Altogether, 90 completed questionnaires were used in the study. Interviews were performed with ten of the supervisees.Data was coded according to principles of grounded theory (Corbin and Strauss, 2015). Our findings revealed that non-disclosure was present in 21% of the sessions. Supervisees conceal dissatisfaction with supervisors and their work, content related to the supervision group, information related to psychotherapeutic work, personal topics and topics related to wider professional activity. The reasons for non-disclosure were: not feeling safe enough in the supervisory relationship or the supervision group, concern for the supervisor, shame and self-criticism. These findings are significant both for the practice and for further research of supervision.