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We aim to estimate the number of people living with HIV and the undiagnosed fraction in Spain, where coverage of the HIV surveillance system has only recently become complete.The reconstruction of all HIV diagnoses and infections was obtained by combining HIV and AIDS surveillance data. The imputation of the diagnoses and back-calculation of the infection incidence are integrated in a Bayesian framework to take into account the uncertainty associated with unavailable data.An estimated 141 000 [95% credible interval (CI) 128 000–155 000] persons were living with HIV by the end of 2013, in Spain and 18% (95% CI 14.3–22.1%) were unaware of it. A similar fraction of undiagnosed infections was obtained in men who have sex with men and heterosexuals (18.8 and 20.1%, respectively), but for injection drug users, this fraction was 3.5%.This study provides the first estimates of the number of people living with HIV and the undiagnosed fraction in Spain, using routine surveillance data. The proposed method could be useful for countries where the geographical coverage of the HIV surveillance system is partial or was completed only recently.