Knowledge of AIDS and its transmission was studied in patients undergoing drug-dependence treatment for opiates and/or cocaine. The study area included all of Spain and was carried out via a questionnaire assessing information about risk practices, HIV serostatus, etc, and 13 true or false questions concerning HIV-transmission knowledge. The main purpose of the present study was to analyse the relationship between the level of information found in patients and factors potentially related to it, including the practice of high-risk activities for HIV transmission, the sources of information and behaviour modification. Findings indicate that poor information was associated with the use of non-sterile needles, anal-penis sexual relations, non-use of condoms, lack of preventive measures in daily cohabitation, and lack of previous drug-dependence treatment. Determining factors of being well-informed were a high level of education, a longer duration of intravenous drug use, and contact with the Health Service as a source of information. These findings partially support the need to implement programs aimed at improving knowledge about HIV in the population of drug users.