The success of modern medicine creates a growing population of those suffering from life-threatening illnesses (LTI) who often experience anxiety, depression, and existential distress. We present a novel approach; investigating MDMA-assisted psychotherapy for the treatment of anxiety in people with an LTI. Participants with anxiety from an LTI were randomized in a double-blind study to receive MDMA (125 mg, n = 13) or placebo (n = 5) in combination with two 8-h psychotherapy sessions. The primary outcome was change in State-Trait Anxiety Inventory (STAI) Trait scores from baseline to one month post the second experimental session. After unblinding, participants in the MDMA group had one open-label MDMA session and placebo participants crossed over to receive three open-label MDMA sessions. Additional follow-up assessments occurred six and twelve months after a participant’s last experimental session. At the primary endpoint, the MDMA group had a greater mean (SD) reduction in STAI-Trait scores, − 23.5 (13.2), indicating less anxiety, compared to placebo group, − 8.8 (14.7); results did not reach a significant group difference (p = .056). Hedges’ g between-group effect size was 1.03 (95% CI: − 5.25, 7.31). Overall, MDMA was well-tolerated in this sample. These preliminary findings can inform development of larger clinical trials to further examine MDMA-assisted psychotherapy as a novel approach to treat individuals with LTI-related anxiety. Trial Registration: clinicaltrials.gov Identifier: NCT02427568, first registered April 28, 2015.