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Background Although men who have sex with men (MSM) are considered at high risk for transmission of sexually transmitted infections, including HIV, there are few studies estimating the population size of MSM in Europe. We used network data from a survey of MSM in four cities to perform successive sampling–population size estimations (SS-PSE) to estimate MSM population sizes. Methods Data were collected in 2013–14 in Bratislava, Bucharest, Verona and Vilnius using respondent-driven sampling (RDS). SS-PSE uses a Bayesian framework to approximate the RDS sampling structure via a successive sampling model and uses the selection order of the sample to provide information about the distribution of network sizes over the population members of MSM. Results We estimate roughly 4600 MSM in Bratislava, 25 300 MSM in Bucharest, 7200 in Verona and 2900 in Vilnius. This represents 2.9% of the estimated adult male population in Bratislava, 2.3% in Bucharest, 2.7% in Verona and 1.5% in Vilnius. The number of MSM living with HIV would roughly be 200 in Bratislava, 4554 in Bucharest, 690 in Verona and 100 in Vilnius. Conclusions Benefits of this method are that no additional information from an RDS survey needs to be collected, that the sizes can be calculated ex post facto a survey and that there is a software programme that can run the SS-PSE models. However, this method relies on having reliable priors. Although many countries are estimating the sizes of their vulnerable populations, European countries have yet to incorporate similar and novel methods. [ABSTRACT FROM AUTHOR]