Although initial rituximab-containing chemotherapies achieve high response rates, indolent B-cell non-Hodgkin lymphoma (B-NHL), such as follicular lymphoma (FL), is still incurable. Therefore, new effective agents with novel mechanisms are anticipated. In this multicentre phase II study, patients with relapsed/refractory indolent B-NHL and mantle cell lymphoma (MCL) received vorinostat 200 mg twice daily for 14 consecutive days in a 21-d cycle until disease progression or unacceptable toxicity occurred. The primary endpoint was overall response rate (ORR) in FL patients and safety and tolerability in all patients. Secondary endpoints included progression-free survival (PFS). Fifty-six eligible patients were enrolled; 50 patients (39 with FL, seven with other B-NHL, and four with MCL) were evaluable for ORR, and 40 patients had received rituximab-containing prior chemotherapeutic regimens. For the 39 patients with FL, the ORR was 49% [95% confidence interval (CI): 32·4, 65·2] and the median PFS was 20 months (95% CI: 11·2, 29·7). Major toxicities were manageable grade 3/4 thrombocytopenia and neutropenia. Vorinostat offers sustained antitumour activity in patients with relapsed or refractory FL with an acceptable safety profile. Further investigation of vorinostat for clinical efficacy is warranted.