OBJECTIVES: Recent outbreaks of the mpox (monkeypox) virus have been detected in dense sexual networks of gay and bisexual men who have sex with men (GBMSM). The objective of this study is to describe and compare the epidemiological and behavioural characteristics, as well as the sexual networks, of GBMSM diagnosed with mild mpox in Spain. METHODS: A prospective case-control study was conducted in Spain from July 2022 to February 2023. The study targeted a key population of GBMSM aged 18 years or older. Study participants were categorised into cases, those who were diagnosed with mpox virus infection; and controls, those who were not diagnosed. We examined and compared the sexual network characteristics of the two groups-mpox-positive (mpox-P) and mpox-negative (mpox-N) egos-using χ2, t-test and Wilcoxon test to examine the differences between the two groups in each section. Finally, we conducted univariable and multivariable logistic regressions to determine the factors associated with mpox infection. RESULTS: Among the 105 participants, 35 (33.3%) were mpox-P. Compared with mpox-N, mpox-P respondents more frequently reported syphilis (mpox-P: 31.4%; mpox-N: 12.9%) and HIV (mpox-P: 45.7%; mpox-N: 18.6%), and mpox-P individuals to have had at least one sexual contact with a confirmed mpox case (mpox-P: 62.5%; mpox-N: 8.3%). In the egocentric network analysis, mpox-P respondents had a higher prevalence of group sex with alters (mpox-P: 18.5%; mpox-N: 8.9%) and one-time sexual partners (mpox-P: 46.1%; mpox-N: 31.7%). Multivariable logistic regressions showed that reporting stranger/client ties (adjusted OR (aOR)=10.3, 95% CI 1.39 to 76.6) with alters, being vaccinated for mpox (aOR=0.07, 95% CI 0.02 to 0.24) and tie strength heterogeneity (aOR=0.01, 95% CI 0.00 to 0.42) were associated with mpox infection. CONCLUSIONS: Our findings highlight the role of demographic, epidemiological and sexual network characteristics in the transmission of mpox virus during the outbreak in Spain. These findings have important implications for future prevention efforts.