Purpose: The study aimed to determine the body composition of Japanese adult survivors with childhood cancer. Methods: Between August 2018 and September 2019, we recruited adults aged ≥18 years who had childhood leukemia/lymphoma. Blood sampling, body composition measurement by bioelectrical impedance analysis (BIA), grip strength test, nutrition consultation, and screen viewing time survey were conducted. Results: We analyzed the data of 81 survivors with a median age of 25 years. The disease profile comprised 51 cases of acute lymphocytic leukemia, 20 acute myelocytic leukemias, and 10 lymphomas; 34 patients had undergone hematopoietic stem cell transplantation (HSCT). The average body mass index (BMI) was 21 kg/m2. Of the patients, 10 (12%) were obese and 18 (22%) were lean. Three patients had metabolic syndromes (MS) and 9 had sarcopenia. There was no significant difference in the frequency of obesity, sarcopenia, and MS in each disease; however, sarcopenia was significantly higher in the transplant group. Further, 30% of obese patients had MS, and 33% of lean patients had sarcopenia. Discussion: Leanness and sarcopenia are common in HSCT survivors, and the possible involvement of transplant-related complications is accordingly suspected. The effects of lifestyle cannot be ruled out, and more cases must be analyzed to examine risk factors. Conclusion: Some young adult survivors with childhood leukemia/lymphoma may have metabolic syndrome (MS) or sarcopenia. Assessments of body composition by BIA for childhood cancer survivors could benefit to detect MS or sarcopenia in their young adulthood.