Background: The Prostate Cancer Research International: Active Surveillance (PRIAS) study is a large international prospective observational AS study that commenced in 2006. Japan has participated in the study (PRIAS-JAPAN) since 2010, and recruitment remains ongoing. The objective of this study is to define clinical and pathological factors predicting reclassification at the time of 1-year repeat biopsy (re-Bx) based on a Japanese cohort forming part of the PRIAS study. Methods: The inclusion criteria for the PRIAS study are as follows: clinical stage T1c/T2, PSA ≤ 10 ng/ml, PSA density (PSAD) < 0.2 ng/ml per milliliter, one or two positive biopsy cores, and Gleason score (GS) ≤ 6 at initial diagnostic biopsy. Baseline clinical characteristics and prostate-specific antigen doubling time (PSADT) at the time of re-Bx were analyzed via multivariate logistic regression with respect to reclassification on the 1-year re-Bx. Results: A total of 711 patients were enrolled in PRIAS-JAPAN by September 2016. Of these, 409 underwent re-Bx at 1-year. The re-Bx acceptance rate was 83.3%. A total of 122 (29.8%) was reclassified whereas 150 (36.7%) had no cancer. Older age, a higher PSAD, a higher positive core rate, and a shorter PSADT were significant predictors of reclassification. Among them, the positive core rate was the strongest predictor for pathological reclassification at 1-year after starting AS. The AS remaining rates at 1, 2, 3, 4 and 5 years were 93.9%, 70.6%, 65.7%, 60.0% and 49.5% respectively Conclusions: An interim analysis of a Japanese AS cohort participating in PRIAS revealed that the positive core rate was strongly associated with reclassification at the 1-year re-Bx. Clinical trial information: UMIN000002874.