Importance: The ongoing COVID-19 pandemic restricts access to care for psychiatric patients. The physical and mental well-being of patients with severe mental illness in the current circumstances is unknown. Objective: To evaluate physical and mental well-being, subjective mental health, and need for updated psychiatric management plans in a sample of patients with severe mental illness during the early stages of the COVID-19 pandemic. Design: Cross-sectional study of structured telephone assessments conducted between April 23 and June 30, 2020. Setting: Regional psychiatric outpatient care centre in Stockholm, Sweden. Participants: Patients who had not been in contact with their psychiatric clinic between April 9 and April 23, 2020. A total of 1071 patients were contacted by phone. Exposures: Occurrence of respiratory symptoms, changes in psychiatric symptoms, and the need for updated psychiatric management plans, as determined by the telephone assessors. Subjective mental health rated 0-100 by patients. Main Outcomes and Measures: Self-rated physical, respiratory and psychiatric symptoms according to a semi-structured interview. Subjective mental health rated on a scale from 0-100. Results: Patients (n = 1071) were on average 45 years old (SD = 16.9), of which 570 (53%) were female. Neurodevelopmental disorders, psychotic disorders, and bipolar disorder were the most common diagnostic categories. The majority of respondents reported no respiratory symptoms (86%), and few reported light (10%) or severe (4%) respiratory symptoms. Similarly, most patients reported no worsening in psychiatric symptoms (81%). For those who reported a worsening of psychiatric symptoms (19%), the psychiatric management plans that were already in place were deemed appropriate in most cases (16.5%), whereas 22 patients (2.5%) reported a worsening of psychiatric symptoms that warranted an earlier or immediate follow-up by their psychiatric clinic. Patients rated their subjective mental health on a 0-100 scale as 70.5 [95% CI 69 - 71.9] on average (n = 841). Response rates to the questions of the structured assessment varied from 79% - 82%. Conclusions and Relevance: The majority of patients reported no respiratory symptoms, no change in psychiatric symptoms and a rather high subjective well-being. Patients in psychiatric care with a mental health care plan experienced stability in the management of their psychiatric symptoms and general well-being, and only a minority were in need of acute support during the early pandemic phase in Stockholm, Sweden.