<p class="abstract">Immune check point inhibitors have demonstrated promising efficacy in patients with recurrent or metastatic nasopharyngeal carcinoma (NPC) in phase I and phase II trials. Early identification of treatment response is important in these patients. This report aimed to document the early intratreatment diffusion weighted magnetic resonance imaging (DW-MRI) findings in NPC patients following treatment with the programmed cell death-1 inhibitor, nivolumab. Two consecutive patients with histologically confirmed recurrent undifferentiated NPC treated with nivolumab were prospectively recruited. Nivolumab was administered at a dosage of 3 mg/kg intravenously every 2 weeks. Patients underwent magnetic resonance imaging examinations at baseline, and at 3 and 5 weeks after commencement of treatment. Intratreatment changes in tumour volume and apparent diffusion coefficient (ADC_mean)were calculated. The endpoints were objective response by response evaluation criteria in solid tumors and survival. In patient 1, an intratreatment ADC increase at 5 weeks corresponded with anatomical tumour volume reduction and a better long-term survival outcome (progression free survival 1.3 years, overall survival 2.9 years). In patient 2, an intratreatment ADC decrease at 5 weeks corresponded to progressive disease and worse outcome (progression free survival 0.0 years, overall survival 0.9 years). Intratreatment ADC changes at 3 weeks were not associated with response outcome. These cases suggest that intratreatment changes in ADC at 5 weeks may potentially predict tumour response in patients treated with nivolumab. Dedicated studies are needed to clarify these findings and fully characterise patterns of treatment related ADC change.</p>