For adopted individuals, understanding the role of birth family is an important part of developing a coherent life narrative. However, there is limited empirical research on this aspect of the adoption experience. We introduce a new construct, birth family thoughts, that captures a sense of curiosity about birth family, and describe the development of an accompanying brief self-report measure, the Birth Family Thoughts Scale (BFTS). Across 4 studies of transnationally adopted Korean American adolescents, emerging adults, and adults who were adopted before the age of 3 (ncombined = 546), we found strong support for a 1-factor structure using exploratory and confirmatory factor analyses, and good internal consistency and test-retest reliability. Convergent validity was generally supported. The BFTS was positively related to measures of adoption- and ethnicity-related constructs, although there were a few inconsistencies between studies and measures. Discriminant validity also was generally supported. We found no evidence for the BFTS being related to a poor adoptive family situation or an indication of psychopathology. We did find some evidence of the BFTS relating to internalizing and externalizing symptoms. Furthermore, while the BFTS was unrelated to travel to Korea, it was correlated with visiting an orphanage in Korea. It was also related to initiating a birth family search in Study 1, but not in Studies 2 or 3. We discuss the importance of considering birth family thoughts across the life span and with other populations, as well as the limitations of the current study including sampling issues inherent in working with hard-to-reach populations.