Based on their transphobic assumption that being transgender or nonbinary is pathological or otherwise undesirable, gender identity change efforts (GICE) attempt to make a person’s gender conform with their sex assigned at birth. While many professional bodies have noted that GICE practices are unethical, there has been little empirical research into the prevalence and correlates of GICE exposure. Counting Ourselves: The Aotearoa New Zealand Trans and Nonbinary Health Survey is a community-based study, which participants completed mostly online. Out of 610 participants who had ever spoken to a health professional about their gender, 19.7%, 95% CI [16.6%, 23.1%], reported GICE exposure, and a further 9.3% [7.2%, 11.9%] did not know. GICE exposure was higher among younger participants. Participants with GICE exposure were more likely than those without such exposure to report psychological distress, nonsuicidal self-injury, suicidal ideation, and suicide attempts (e.g., suicidal ideation OR = 2.39). GICE partially mediated the effect of family rejection on mental health, and internalized transphobia partially mediated the effect of GICE on mental health. These correlates between GICE and mental health replicate recent findings from the U.S. Trans Survey, and the mediation analyses help to understand potential causal mechanisms underlying these correlations. Although our findings are limited by being a convenience sample, they are consistent with the hypothesis that GICE exposure is harmful to transgender or nonbinary people’s mental health. Moreover, these findings support moves by many professional bodies to emphasize that GICE is unethical and the legal steps taken by a growing number of jurisdictions to ban such practices. (PsycInfo Database Record (c) 2021 APA, all rights reserved)