This study explores the knowledge, attitudes and perception of tuberculosis (TB) and how they affect adherence to preventive treatment. A qualitative study of 24 subjects taking preventive therapy for TB was undertaken. Interviews were conducted in English using a structured questionnaire in a private room adjoining the outpatient clinic before the subject's appointment. The questionnaire included; (i) demographic details; (ii) background information on TB; (iii) knowledge and perception of TB and chemoprophylaxis. Data was analysed descriptively and thematically and subjected to statistical evaluation where appropriate. Outcome was assessed >6 months after the start of preventive treatment. The sample was representative of the ethnic mix of those attending the clinic. Knowledge of TB-was gained outside the family and covered transmission but few symptoms of active TB were recognized. The perceived threat from TB was high, although the estimated risk was low. Knowledge of preventive therapy exceeded the general knowledge of TB, although the latter was associated with better adherence. Most denied knowledge of the risk of hepatitis from isoniazid. Defaulters failed to attend their first appointment, attributed more side effects to isoniazid and perceived a longer waiting time in clinic. Adherence is better with a general knowledge of TB and attendance at the first appointment and could be improved by a single daily tablet, warnings about dizziness and discussion of the difficulties of taking a 6 month course of treatment.