Romidepsin belongs to the group of histone deacetylase inhibitors (HDI). Although HDIs are a promising class of chemotherapeutic agents in T-cell lymphomas, their exact mechanism of anti-tumour activity is not yet fully understood. The US Food and Drug Administration approved romidepsin for the treatment of relapsed or refractory cutaneous T-cell lymphoma and relapsed or refractory peripheral T-cell lymphoma (PTCL) in 2009 and 2011, respectively. In Europe, romidepsin is not yet approved but under marketing authorisation review for the treatment of relapsed or refractory PTCL patients. Focusing on the indication of interest of this horizon scanning assessment, i.e. relapsed or refractory PTCL, two single-arm phase II studies investigating the response of patients to romidepsin were included. Overall, 175 patients with a median age of 59 to 61 (range 20 to 84) were included in these studies. The overall response rates (ORR) observed were 25% to 38% of which 15% to 18% were complete responses and 11% to 20% were partial responses. The median durations of response reported were 16.6 and 8.9 months in these two trials. Generally, most adverse events (AEs) were considered to be mild to moderate in severity. Infections were the most commonly observed severe AEs. Across both studies, 10 patients died while on study. One death was reported to be study-drug related. Overall, the small population included and the single-arm design, as well as the primary efficacy endpoint 'response rate' do not allow full characterisation of the safety and efficacy profile of romidepsin, based on the available data.