Control of vaccine preventable diseases (VPDs) is a challenge for healthcare systems. Different studies highlighted the suboptimal immunization of internationally adopted children (IAC). To evaluate the immunization status against measles, mumps, rubella (MMR), and varicella (V) in a large cohort of IAC, data at first screening visit of all IAC (<18 years) consecutively referred to Meyer Children’s University Hospital (Florence, Italy) from 2009 to 2018 were collected and analyzed. In total, 1927 children (median age: 5.99 years, interquartile range: 3.33–8.21) were enrolled. More than half of IAC were unprotected against MMR-V. The reliability of the vaccination documentation of the country of origin was poor, since more than a quarter of the IAC serologically tested were not protected against MMR-V, despite the vaccination documentation attesting previous vaccination. This was significantly more pronounced in children aged 15–18 years and in those originating from Africa. High rate of discordant serological results/documentation brings up questions regarding the optimal management of IACs, and suggests a rapid, careful, and complete assessment of immunization status timely after IAC’s arrival. Serological testing of IAC of all ages followed by vaccination of seronegative children should be provided.