Lichenoid drug eruption is a rare form of delayed type drug eruptions. Among antituberculosis drugs, ethambutol is one of the most common causative drugs to induce lichenoid drug eruptions and cycloserine has been reported known as a rare causative drug of the lichenoid drug eruptions. The lymphocyte transformation test (LTT) is a safe and reliable diagnostic procedure for drug allergy, but it is rarely used for diagnosis of lichenoid drug eruptions. In the current case, we performed an LTT and successfully con. rmed cycloserine as the offending drug in the patient who were treated with many antituberculosis drugs. This suggests that the cycloserine should be considered as a possible causative drug of lichenoid drug eruption and an LTT could be an option for the diagnosis of lichenoid drug eruption due to cycloserine.