Background: The pRB-dependent cell cycle checkpoint is altered in the vast majority of anaplastic oligodendrogliomas (AO), either by homozygous deletion or by hypermethylation of CDKN2A and/or CDKN2B, or by amplification and/or overexpression of CDK4. Palbociclib is an oral inhibitor of CDK4 and 6 that has already been shown to be highly active in breast cancer. Methods: We conducted a multicenter, open-label, phase II trial evaluating efficacy and safety of Palbociclib in patients with AO that progressed to radiotherapy and more than one chemotherapy regimen containing Temozolomide and/or Lomustine. Inclusion criteria included: histologically and molecularly confirmed grade III oligodendroglioma (WHO 2016 classification, IDH1/2 mutation and 1p/19 codeletion were mandatory), recurrence after radiotherapy and 1 or 2 chemotherapy regimens and conserved RB protein expression by immunohistochemistry (IHC). Patients were treated with Palbociclib 125 mg/daily 3 weeks on/1off. The primary objective of the study was progression-free survival at 6 months (6M-PFS). Results: Between October 2015 and September 2018, 34 patients were enrolled across ten hospitals. The study was stopped early secondary to lack of efficacy, with 74% of evaluable patients progressing within 6 months. Number of patients alive and free from progression at 6 months after the enrollment was 9 (26%) out of the first 34 patients, below the minimum number required (18 out of 40) to consider Palbociclib as an active drug in this population. With a median follow-up of 11.2 months, the median PFS was 3 months (95% CI: 2.5-3.5 months). Median overall survival (OS) was 23.1 months (95% CI: 17.2-25 months). There were no partial or complete responses and only 11 patients (32%) achieved stable disease as best response. Palbociclib was well tolerated with neutropenia (Grade 3 or 4: 40%) and thrombocytopenia (Grade 3 or 4: 15%) as the most common adverse effects (AEs). Both AEs had no significant impact since there were no episodes of febrile neutropenia or bleeding. Conclusions: Despite the good tolerance and drug exposure, Palbociclib monotherapy did not show favorable activity in recurrent AO. Clinical trial information: NCT02530320