Many couples start living together or decide to break up without an official marriage or divorce. A database containing information derived from registers on start and end dates of cohabitation relationships was not readily available yet. However, building such a database appeared to be possible by using longitudinal register data on marriages and births, partnerships for income taxes and social security benefits, moving to an address together and on where people live, in combination with an imputation strategy applied to ‘ambiguous couples’, being two persons living together at one address but of whom we did not have any other officially registered information about their relationship. The imputation strategy is also used to reduce the bias that would arise in the database when only using the longitudinal register data. For couples that recently started living together at the same address, less register information about their common future is available. As a result, less of them could be directly designated as a cohabiting couple. Having created this file, we can study relationships over time at the individual level. Furthermore, since the file can be linked to other registers, an abundance of additional research opportunities into the phenomenon of cohabitation has become available.