Résumé : Background: We evaluate differences in timing of cART (combined antiretroviral treatment) initiation by geographical origin in male and female HIV-positive patients in the Collaboration of Observational HIV Epidemiological Research Europe, a large European Collaboration of HIV Cohorts. Methods: We included individuals recruited in Western Europe between January 1997 and March 2013, with known geographical origin and at least 1 CD4þ cell count measurement while cART-naive. Timing of cART was assessed through modified time-to-event methods, in which a scale of CD4þ cell counts was used instead of time, with cART being the outcome. We estimated the median CD4þ cell count at cART initiation (estimated CD4þ levels at which the probability of having started cART is 50%) using Kaplan-Meier and adjusted hazard ratios of cART initiation using Cox regression. Results: Of 151 674 individuals, 110 592 (72.9%) were men. Median (95% confidence interval) CD4þ cell count falls far below 250 cells/ml in all groups and was lowest in sub-Saharan African [SSA: 161 (158-167)], Caribbean men [161 (150-174)] and in Asian women [Asian Continent and Oceania: 185 (165-197)]. Among men, the adjusted probability of cART initiation was lower in migrants compared with natives, but differences depended on initial CD4þ cell count. For example, in the group with more than 500 CD4þ at recruitment, they were 45% (36-53%), 30% (17-40%) and 25% (19-30%) lower for Caribbean, Eastern European and SSA men, respectively. In women, no meaningful differences were observed between natives and most migrant groups. However, SSA women had a 31% (24-38%) higher probability of cART initiation when recruited at a CD4þ more than 500 cells/ml and 9% (4-14%) lower when recruited at CD4þ less than 100 cells/ml. Conclusion: Most migrant men initiate cART at lower CD4þ cell count than natives, whereas this does not hold for migrant women.