OBJECTIVES: Lockdown measures in response to the coronavirus disease 2019 (COVID-19) pandemic can have serious mental health effects on the population, especially in vulnerable groups, such as those living in poor socio-economic conditions, those who are homeless, migrant workers and asylum seekers/refugees. In addition, these vulnerable groups frequently have greater difficulty accessing health services and in treatment adherence. The aim of this study is to estimate the impact of the COVID-19-related lockdown on service utilisation and follow-up adherence in an Italian mental health outpatient service for migrants and individuals in socio-economic difficulties. STUDY DESIGN: The design of this study is a retrospective cross-sectional study. METHODS: All patients who visited the mental health outpatient service in the months of February and March in the years 2017-2020 were included in the study. To compare service utilisation before and after the lockdown, the number of patients who visited the mental health outpatient service for psychiatric interview were recorded. Follow-up adherence was calculated as the percentage of patients who visited in February and subsequently attended a follow-up visit in March of the same year. RESULTS: The number of patients who visited the outpatient service between February 2017 and February 2020 was continuously increasing. In March 2020, fewer patients visited the service for psychiatric interview, in line with the introduction of lockdown measures. In addition, the number of the patients who visited in February 2020 and returned for their follow-up visits in March 2020 declined from approximately 30% over the same months in 2017-2019 to 17.53% in March 2020. CONCLUSIONS: The lockdown-related reduction in numbers of patients accessing the mental health service makes it difficult to help vulnerable populations during a period of time in which their mental health needs are expected to increase. Moreover, the reduction seen in follow-up compliance increases the risk of treatment discontinuation and possible relapse. Proactive alternative strategies need to be developed to reach these vulnerable populations.