BACKGROUND: This aim of this investigation was to track changes and risk factors for mental health outcomes during state-mandated quarantine in Brazil. METHODS: Adults residing in Brazil (N.=360, 37.9 years old, 68.9% female) were surveyed at the start of quarantine and approximately three weeks later. Outcomes assessed included perceived stress, state anxiety and symptoms of depression. Aside from demographics, behaviours and attitudes assessed included exercise, diet, use of tele-psychotherapy and a number of COVID-19 related risk factors, such as perceived risk of COVID-19, information overload, and feeling imprisoned. RESULTS: Overall, all mental health outcomes worsened from time 1 to time 2, although there was a significant gender x time interaction for stress. At time 2, 9.7% of the sample reported stress above the clinical cut-off (2 SD above mean), while 8.0% and 14.9% were above this cut-off for depression and state anxiety, respectively. In repeated measures analysis, female gender, worsening diet and an excess of COVID-19 information was related to all mental health outcomes. Positive dietary changes were associated with decreases in depression and anxiety. Mental health was worst for those reporting 0 days/week of exercise. Those who did aerobic exercise did not have a significant increase in depression. Use of tele-psychotherapy predicted lower levels of depression and anxiety. In multiple regression, anxiety was predicted by the greatest number of COVID-19 specific factors. CONCLUSIONS: Mental health outcomes worsened for Brazilians within the first month of quarantine and these changes are associated with a variety of risk factors.