The aim of this phase II study was to investigate the efficacy and tolerability of liarozole, a novel benzimidazole derivative, in non-small cell lung cancer (NSCLC). Liarozole 300 mg twice daily orally was evaluated in 14 patients with stage IIIB and IV NSCLC. 8 patients had received prior treatment with chemotherapy and/or radiotherapy. WHO toxicity grading and response criteria were used. Liarozole was well tolerated. Grade 2 toxicities included alopecia (1 patient), dermatological toxicity (5 patients), dry mouth (2 patients) and nausea and vomiting (2 patients). Leukocytosis was seen in 5 patients, including 2 cases with an elevated white cell count pretreatment. Liarozole was discontinued in 1 patient who developed intolerable progressive pruritus associated with an erythematous rash. No objective tumour response was seen, all 14 patients developing progressive disease within 4 months of commencing treatment. Liarozole was well tolerated but was ineffective as single agent therapy in the management of NSCLC. The side-effect profile was compatible with inhibition of retinoic acid degradation.