STUDY OBJECTIVE--To determine whether HIV positive intravenous drug users (IVDUs) who were receiving outpatient treatment for opiate and cocaine abuse or dependence used practices aimed at reducing the spread of HIV. DESIGN--Cross sectional study of behaviour and HIV serostatus in IVDUs. SETTING--A nationwide sample, from 83 health centres for outpatient treatment, stratified by autonomous regions. PARTICIPANTS--Altogether 1074 IVDUs were recruited. HIV serostatus could be verified in 738 (68.7%) of these. MEASUREMENTS AND MAIN RESULTS--Crude and adjusted odds ratios and their 95% confidence intervals were estimated to assess the association between HIV serostatus and behavioural changes. In their daily interactions with other members of the same household, seropositive subjects more frequently used preventive methods aimed at avoiding transmission than seronegative patients. Treatment for abuse or dependency before the current regimen had a greater impact in HIV positive than HIV negative subjects in terms of abstaining from risk behaviours. There was a significant trend toward lower drug consumption in HIV positive subjects, and the number of seropositive and seronegative IVDUs who stopped injecting their drugs was significantly higher among the former. Seropositive subjects were also more likely to stop sharing drug injecting equipment and to change their sexual habits; they reported an increased consistent use of condoms. CONCLUSIONS--HIV positive IVDUs were more likely to change their risk behaviours than their HIV negative counterparts.