Active surveillance (AS) aims to reduce overtreatment of low-risk prostate cancer (PC). Incorporating multiparametric magnetic resonance imaging (mp-MRI) and MR-guided biopsy (MRGB) in an AS protocol might contribute to more accurate identification of AS candidates. To evaluate the value of 3T mp-MRI and MRGB in PC patients on AS at inclusion and after 12-mo follow-up. Patients with cT1c-cT2 PC, prostate-specific antigen (PSA) ≤10ng/ml, PSA density <0.2ng/ml/ml, and Gleason scores (GSs) of ≤6 and ≤2 positive biopsy cores were included and followed in an AS protocol including mp-MRI and MRGB. The mp-MRI and MRGB were performed at <3 and 12 mo after diagnosis. Reclassification was defined as GS >6, >2 positive cores at repeat transrectal ultrasound-guided biopsy (TRUSGB), presence of PC in >3 separate cancer foci upon both MRGB and TRUSGB, or cT3 tumor on mp-MRI. Reclassification rates, treatment after discontinuation, and outcome on radical prostatectomy after discontinuing AS were reported. Uni- and multivariate analyses were performed to identify predictors of reclassification after 1 yr. From 2009 to 2013, a total of 111 of 158 patients were consecutively and prospectively included. Around initial diagnosis, 36 patients were excluded from the study protocol; mp-MRI+MRGB reclassified 25/111 (23%) patients, and 11 patients were excluded at own request. Reasons for reclassification were as follows: GS upgrade (15/25, 60%); cT3 disease (3/25, 12%); suspicion of bone metastases (1/25, 4%); and multifocal disease upon MRGB (6/25, 24%). Repeat examinations after 1 yr showed reclassification in 33/75 patients (44%). Reasons were the following: GS upgrade upon TRUSGB (9/33, 27%); volume progression upon TRUSGB (9/33, 27%); cT3 disease upon mp-MRI (1/33, 3%); GS upgrade upon MRGB (1/33, 3%); volume progression upon MRGB (1/33, 3%); multifocal disease upon MRGB (2/33, 6%); and upgrade or upstage upon both TRUSGB and MRGB (10/33, 30%). On logistic regression analysis, the presence of cancer at initial mp-MRI and MRGB examinations was the only predictor of reclassification after 1 yr (odds ratio 5.9, 95% confidence interval 2.0-17.6). Although mp-MRI and MRGB are of additional value in the evaluation of PC patients on AS, the value of mp-MRI after 1 yr was limited. As a considerable percentage of GS ≥7 PC after 1 yr was detected only by TRUSGB, TRUSGB cannot be omitted yet. More aggressive tumors are detected if low-risk prostate cancer patients are additionally monitored by magnetic resonance imaging. However, some high-grade tumors are detected only by transrectal ultrasound-guided biopsy.