Seventy-seven (71%) of a group of 109 attenders at an out-patient drug treatment service reported that they had used heroin in the 90 days before interview, of whom 24 (31%) had used every day. Daily users were more likely to explain their use in terms of needing to curb withdrawals than were occasional heroin users. The latter group were more likely to report availability as a reason for use. From a clinical perspective, it is likely that those who use opportunistically are less likely to change their use as a function of clinical responses (e.g. higher methadone dose) than are those whose use is motivated by the attempt to curb withdrawal symptoms.