Eighty-eight drug addicts from the "BAN Center" in Torre Annunziata (Naples) and 88 normal subjects pair-matched for age and sex were tested for IgG to human immunodeficiency virus (HIV), herpes simplex virus (HSV) type 1 and 2 and cytomegalovirus (CMV). A high prevalence of subjects with antibodies to HSV-1 and CMV (80.7% and 65.9%) were recorded in the control group testifying to the high level of these infections in Campania. Prevalences were higher in drug addicts, and drug abuse was identified as a risk factor for the acquisition of CMV infection (odds ratio = 2.3). Moreover, drug addiction is also a risk factor for HSV-2 and HIV infection as demonstrated by the observation that drug abusers were anti-HSV-2 (9.1 vs. 1.1%, odds ratio = 6.16) or anti-HIV (11.4 vs. 0%, odds ratio = 23.6) positive more frequently than normal controls. Thus, drug addiction is a risk factor for the acquisition of HIV, HSV-2 and CMV infections. This is probably due to similar habits, frequent among drug addicts from our geographic area and uncommon in the normal population, such as tattooing, needle-sharing needlestick and unsafe sex. Some of these habits, such as unsafe sex and tattooing, seem to be, per se, risk factors for the acquisition of both HIV and CMV infections. The data also suggest that HIV infection was probably introduced in Campania more recently than in northern and central Italy where the prevalence of anti-HIV positive cases among drug addicts is definitely higher.