Data from a randomized, open label, phase II/III clinical trial investigating the efficacy of pembrolizumab versus docetaxel in patients with advanced-stage non-small-cell lung cancer demonstrate a modest, but significant, improvement in overall survival in the pembrolizumab group — although, no improvements in progression-free survival were observed. When tested only in the subgroup of patients in which PD-L1 is detected in at least 50% of tumour cells, however, pembrolizumab resulted in a much greater overall survival benefit, in addition to a significant improvement in progression-free survival, thus demonstrating the increasing importance of careful selection of clinical trial cohorts.