We reviewed reactions previously reported in patients treated with isoniazid, who ate certain fish and cheeses. We observed similar reactions in two patients after they ingested cheese and wine. Isoniazid is an inhibitor of both monoamine and diamine oxidases, which contribute to the metabolism of histamine that may be present in some fish and cheeses. Monoamine oxidase also acts in the metabolism of tyramine, present in some cheeses and wines. Reactions reported after eating fish or cheese, in patients treated with isoniazid, are similar in that both are characterized by headache, palpitations, skin flushing, nausea, vomiting, and pruritus. Reactions after fish have not been associated with increased blood pressure, whereas those following cheese ingestion frequently result in modest increases in blood pressure. Patients treated with isoniazid should be alerted to the possibility of reactions after eating certain foods.