Background: Bleomycin, etoposide, and cisplatin (BEP) given 3-weekly x 4 remains standard 1st line chemotherapy for intermediate or poor risk metastatic GCT. Accelerating standard regimens by shortening the cycle length to 2-weekly improved cure rates in other cancers. P3BEP will determine effects of accelerated versus standard BEP in this setting. This is the first international, randomized trial of chemotherapy for intermediate and poor-risk metastatic GCT to include adults and children of both sexes. Methods: This open label, randomized, phase 3 trial is conducted seamlessly in 2-stages. The primary endpoint for stage I (n = 150) was favourable response; and for stage 2 (n = 500) is progression free survival at 2 years (PFS2y). These sample sizes provide > 80% power with a 2-sided type I error rate of 5% to detect an absolute improvement of 21% in the favourable response rate (stage 1) and of 7% in the PFS2y (stage 2). The target population is males and females aged 11 to 45 with intermediate- or poor-risk metastatic GCT of the testis, ovary, retroperitoneum, or mediastinum. Participants are randomized (1:1) to 4 cycles of standard BEP (q3w) or accelerated-BEP (q2w) with cisplatin 20mg/m² D1-5, etoposide 100mg/m² D1-5, bleomycin 30 KIU weekly x 12, and pegylated G-CSF D6 or filgrastim daily (Figure 1). Study assessments occur at 30 days after completing chemotherapy, 6 months from randomisation, and after completion of all post-chemotherapy treatments (e.g. surgery). Tumour and blood samples are collected for translational substudies. Progress - As of Jan 2023, 241 participants have been recruited from 22 ANZ sites, 16 UK sites (led by Cambridge Clinical Trials Unit), and 55 USA sites (led by Children’s Oncology Group). The first planned interim analysis for safety (n = 76) identified no safety concerns. The stage I analysis of safety and activity (response rate) for the 1^st 150 patients was reviewed by the IDSMC, which recommended continuation of the trial as per protocol. Clinical trial information: NCT02582697.