A descriptive multicentre study based on laboratory data was carried out in patients with culture-confirmed tuberculosis (TB) who were cared for between September 1995 and August 1996 in 14 general hospitals with at least one human immunodeficiency virus (HIV) ward, a hospital specializing in TB or a large chest clinic, all in the Lombardy region of Italy. For each culture positive for Mycobacterium tuberculosis, the results of susceptibility tests to the five first-line anti-TB drugs were collected; other information collected included: the patient's name, the ward/service where the patient was staying and the characteristics of the culture (biological sample, date of arrival and type of mycobacterium isolated). Foreign-born persons were identified through their names; acquired immune deficiency syndrome (AIDS) patients were identified through record linkage with the National AIDS Registry. Given that only one-quarter of the laboratories performed pyrazinamide-susceptibility tests, resistance to this drug was not analysed. Of the isolates tested, 28.1% were resistant to at least one drug, i.e. single-drug-resistant (SDR) and 12.9% were multidrug-resistant (MDR). The frequency of SDR and MDR strains showed considerable variation by healthcare centre (range 0.0-94.3% and 0.0-37.1%, respectively). In three hospitals, the time of occurrence and the susceptibility pattern of the MDR-TB isolates indicated that clusters of the disease occurred. The frequency of both SDR and MDR strains was significantly higher among AIDS patients (48.8% and 23.6%, respectively) than among foreign-born persons (30.2% and 6.9%) or persons belonging to no known risk group (98.3% and 11.3%). The frequency of drug resistance observed in this study is much higher than that reported in other European surveys and is comparable only to that observed in New York, before the implementation of an effective control programme. Acquired immune deficiency syndrome patients are at very high risk of multidrug-resistant tuberculosis: effective containment and infection control practices should be rigorously implemented to prevent the occurrence of this alarming phenomenon.