BACKGROUND:Knowledge about the health impacts of the absence of health insurance for migrants with precarious status (MPS) in Canada is scarce. MPS refer to immigrants with authorized but temporary legal status (i.e. temporary foreign workers, visitors, international students) and/or unauthorized status (out of legal status, i.e. undocumented). This is the first large empirical study that examines the social determinants of self-perceived health of MPS who are uninsured and residing in Montreal.METHODS AND FINDINGS:Between June 2016 and September 2017, we performed a cross-sectional survey of uninsured migrants in Montreal, Quebec. Migrants without health insurance (18+) were sampled through venue-based recruitment, snowball strategy and media announcements. A questionnaire focusing on sociodemographic, socioeconomic and psychosocial characteristics, social determinants, health needs and access to health care, and health self-perception was administered to 806 individuals: 54.1% were recruited in urban spaces and 45.9% in a health clinic. 53.9% were categorized as having temporary legal status in Canada and 46% were without authorized status. Regions of birth were: Asia (5.2%), Caribbean (13.8%), Europe (7.3%), Latin America (35.8%), Middle East (21%), Sub-Saharan Africa (15.8%) and the United States (1.1%). The median age was 37 years (range:18-87). The proportion of respondents reporting negative (bad/fair) self-perception of health was 44.8%: 36.1% among migrants with authorized legal status and 54.4% among those with unauthorized status (statistically significant difference; p<0.001). Factors associated with negative self-perceived health were assessed using logistic regression. Those who were more likely to perceive their health as negative were those: with no diploma/primary/secondary education (age-adjusted odds ratio [AOR]: 2.49 [95% CI 1.53-4.07, p<0.001] or with a college diploma (AOR: 2.41 [95% CI 1.38-4.20, p = 0.002); whose family income met their needs not at all/a little (AOR: 6.22 [95% CI 1.62-23.85], p = 0.008) or met their needs fairly (AOR: 4.70 [95% CI 1.21-18.27], p = 0.025); with no one whom they could ask for money (AOR: 1.60 [95% CI 1.05-2.46], p = 0.03); with perception of racism (AOR: 1.58 [95% CI 1.01-2.48], p = 0.045); with a feeling of psychological distress (AOR: 2.17 [95% CI 1.36-3.45], p = 0.001); with unmet health care needs (AOR: 3.45 [95% CI 2.05-5.82], p<0.001); or with a health issue in the past 12 months (AOR: 3.44 [95% CI 1.79-6.61], p<0.001). Some variables that are associated with negative self-perceived health varied according to gender: region of birth, lower formal education, having a family income that does not meet needs perfectly /very well, insalubrious housing, not knowing someone who could be asked for money, and having ever received a medical diagnosis.CONCLUSIONS:In our study, almost half of immigrants without health insurance perceived their health as negative, much higher than reports of negative self-perceived health in previous Canadian studies (8.5% among recent immigrants, 19.8% among long-term immigrants, and 10.6% among Canadian-born). Our study also suggests a high rate of unmet health care needs among migrants with precarious status, a situation that is correlated with poor self-perceived health. There is a need to put social policies in place to secure access to resources, health care and social services for all migrants, with or without authorized status.