Two representative samples of males first notified as addicts during 1969, one by hospitals and the other by penal establishments, were followed up for five years. It was found that 95 per cent of the hospital sample, but only 30 per cent of the penal sample, were addicted to opiates at some time during this follow-up, as judged by Home Office records. Notification may depend as much upon the setting and circumstances in which doctors see their patients as on their clinical judgement of a ‘notifiable’ case of addiction.