Background: We have previously reported the results of a phase II clinical trial evaluating the safety and efficacy of palbociclib for the treatment of patients with unresectable germ cell tumors, including mature teratoma, teratoma with malignant transformation, and non-teratomatous tumors. In an effort to better assess the efficacy of palbociclib, we now report long-term follow-up of a more homogeneous population of patients with unresectable mature teratoma treated on the phase II clinical trial, with an emphasis on clinically-relevant endpoints for this incurable patient population. Methods: This was a retrospective analysis with long-term follow-up of the patient cohort with mature teratoma treated on the phase II study of palbociclib for the treatment of retinoblastoma protein-expressing germ cell tumors. Patient clinical data was obtained from the medical records and from communication with the enrolled patients and referring medical providers. Major medical events for the treatment of germ cell tumor, including before, during, and after study treatment, were recorded. Clinical events of interest included the initiation of systemic therapy, radiation therapy, surgical debulking, or other invasive procedures. Study endpoints included pre-study and post-study clinical event rates, event-free survival, and radiographic progression-free survival. Results: Long-term follow-up was obtained for 12 patients with mature teratoma. The median pre-study follow-up time was 19.7 months, and the median post-study follow-up time was 38.0 months. The median number of palbociclib treatment cycles was 11. The pre-study clinical event rate was 2.27 events/year (95% CI 1.66 – 3.13), and the post-study event rate was 0.62 events/year (95% CI 0.36 – 1.09). The median progression-free survival was 5.3 months (95% CI 1.8 – 22.6), and the median event-free survival was 16.2 months (95% CI 3.0 – 24.3). Conclusions: The initiation of palbociclib resulted in a clinically-meaningful delay in disease-related major clinical events and radiographic progression. These findings lend further support to the therapeutic activity of CDK4/6 inhibition in this incurable patient population.