OBJECTIVE: To determine whether indeterminate (Likert-score 3/5) peripheral zone (PZ) multi-parametric MRI (mpMRI) studies are classifiable by Prostate-Specific Antigen (PSA), PSA density (PSAD), PI-RADS_v2 rescoring and morphological MRI features. METHODS: Men with maximum Likert-score 3/5 within their PZ were retrospectively selected from 330 men who prospectively underwent prostate mpMRI (3T) without an endorectal coil, followed by twenty-zone trans-perineal template prostate mapping biopsies ± focal lesion-targeted biopsy. PSAD was calculated using pre-biopsy PSA and MRI-derived volume. Two readers A and B independently assessed included men with both subjective Likert-score and PI-RADS_v2. Both readers then classified mpMRI morphological features in consensus. Men were divided into two groups: significant cancer (≥Gleason 3+4) or insignificant cancer (≤Gleason 3+3)/no cancer. Comparisons between groups were made separately for PSA&PSAD using Mann-Whitney test and morphological descriptors with Fisher's exact test. PI-RADS_v2 and subjective Likert assessment were descriptively compared and percentage inter-reader agreement calculated. RESULTS: 76 men were eligible for PSA&PSAD analyses, 71 for PI-RADS scoring, and 67 for morphological assessment (excluding significant image artefacts). Unlike PSA (p=0.915), PSAD was statistically different (p=0.004) between the significant 0.19 ng/ml2 (IQR: 0.13-0.29) and non-significant/no cancer 0.13 ng/ml2 (IQR: 0.10-0.17) groups. Presence of mpMRI morphological features wasn't significantly different between groups. Subjective Likert assessment discriminated patients with significant cancer better than PI-RADS_v2. Inter-reader percentage agreement was 83% for subjective Likert-scoring and 56% for PI-RADS_v2. CONCLUSION: PSAD may categorize presence of significant cancer in patients with Likert-scored 3/5 PZ mpMRI findings. Advances in knowledge: PSAD may be used in indeterminate PZ mpMRI to guide decisions between biopsy versus monitoring.