We study rental income tax compliance using novel third-party information and a large-scale randomized field experiment informing taxpayers about the usage of this information in tax enforcement. The third-party information is register data on the ownership of apartments and tenants. In the RCT tax authorities send treatment letters notifying potential landlords of stricter tax enforcement and the use of third-party information. We find that the letters caused an increase in the propensity to report rental income and an increase in the amount of reported rental income net of expenses. Our research design also allows us to identify members of ownership networks and analyze spillover effects in tax enforcement within them. We find positive reporting spillovers. We do not find evidence of any real effects in housing markets.