Aim Both being underweight and overweight can lead to reduced activity of daily living, which subsequently can require long‐term care. The aim of the present study was to clarify the association between underweight/overweight and the subsequent risk of long‐term care introduction. Methods We tracked the data of long‐term care insurance for 1580 men and women aged ≥65 years who had participated in the official population‐based health check‐up program in 2001 in Tsunan town and Sekikawa village, Japan. The health check‐up data and medical expenditure data for the fiscal year 2001 were used as baseline data. Participants were classified into underweight (body mass index (BMI) <18.5 kg/m2), normal range (BMI = 18.5 to <25.0 kg/m2) and overweight (BMI ≥25.0 kg/m2); the normal range was used as a reference category in Cox proportional hazards models. Results During the average 5.8 years of follow up, 156 participants were identified to start using long‐term care services. Among the young‐old elderly (65–74 years‐of‐age), underweight was significantly associated with the risk of long‐term care introduction (multivariable‐adjusted HR 4.26, 95% CI 1.69–10.72), whereas overweight was not (multivariable‐adjusted HR 1.45, 95% CI 0.69–3.06). Neither underweight nor overweight were significantly associated with long‐term care introduction among the old‐old elderly (≥75 years‐of‐age). Conclusions Underweight could be a good predictor of long‐term care introduction in the young‐old elderly. We should pay attention to underweight in the elderly, as it might be a manifestation of some physical or mental problems related to future long‐term care introduction. Geriatr Gerontol Int 2014; 14: 328–335.