© 2020, The Author(s). Objectives: Evidence-based mindfulness programs have well-established benefits, but the potential for harmful effects is understudied. We explored the frequency and severity of unpleasant experiences and harm in two nonclinical samples participating in an adaptation of mindfulness-based cognitive therapy (MBCT) for the general population. Methods: Study 1 included 84 schoolteachers; study 2 included 74 university students. Both studies were uncontrolled. Participants completed self-report questionnaires about psychological symptoms before and after the 8-week mindfulness course. After the course, they responded to a survey designed for this study that included Likert ratings and free-text questions about unpleasant experiences and harm. All data were collected online. Results: In both samples, about two-thirds of participants reported unpleasant experiences associated with mindfulness practice during the course. Most participants (85–92%) rated these experiences as not at all or somewhat upsetting; some indicated that difficult experiences led to important learning or were beneficial in some way. The proportion of participants reporting harm from the mindfulness course ranged from 3 to 7%. The proportion showing reliable deterioration on symptom questionnaires ranged from 2 to 7%. Those reporting harm and those showing reliable deterioration on questionnaires were largely separate subgroups; only one participant fell in both. Conclusions: Findings highlight the need for mindfulness teachers to manage expectations about benefits and difficulties that may occur in mindfulness-based programs and to work skilfully with participants experiencing difficulties. Experiences of harm may not be captured by symptom questionnaires and should be explicitly assessed in other ways.