Purpose We designed and built dedicated active MR-tracked (MRTR) stylets. We explored the role of MRTR in a prospective clinical trial. Methods and materials Eleven gynecologic cancer patients underwent MRTR to rapidly optimize interstitial catheter placement. MRTR catheter tip location and orientation were computed and overlaid on images displayed on in-room monitors at rates of 6–16 frames-per-second. Three modes of actively-tracked navigation were analyzed: coarse navigation to the approximate region around the tumor; fine-tuning, bringing the stylets to the desired location; pull-back, with MRTR stylets rapidly withdrawn from within the catheters, providing catheter trajectories for radiation treatment planning (RTP). Procedure Catheters with conventional stylets were inserted, forming baseline locations. MRTR stylets were substituted, and catheter navigation was performed by a clinician working inside the MRI bore, utilizing monitor feedback. Results Coarse navigation allowed repositioning of the MRTR catheters tips by 16 mm (mean), relative to baseline, in 14 ± 5 s/catheter (mean ± std). The fine-tuning mode repositioned the catheter tips by a further 12 mm, in 24 ± 17 s/catheter. Pull-back mode provided catheter trajectories with RTP point-resolution of ~1.5 mm, in 1–9 s/catheter. Conclusions MRTR-based navigation resulted in rapid and optimal placement of interstitial brachytherapy catheters. Catheters were repositioned compared to the initial insertion without tracking. In pull-back mode, catheter trajectories matched CT-precision, enabling utilization for RTP.