Data gathered through the Latium HIV Surveillance System were used in conjunction with a compartmental mathematical model to describe the transmission dynamics of HIV-1 in Italy. In the Latium region, as in the rest of Italy, fewer than 1 in 5 cumulative cases of AIDS are attributable to male homosexual transmission, while 55-60 per cent of the cases have been observed among intravenous drug users (IVDUs). Moreover, the number of non-drug-using heterosexual cases is increasing (14 per cent of cumulative cases). Anonymous notification of positive HIV-1 tests, mandatory in Latium since 1985, were used to produce a time series of new HIV-1 diagnoses; just over 400 new cases of infection per quarter were diagnosed from mid 1989 to mid 1990, with no evidence of increasing incidence. A minimum of 6009 and a maximum of 10,000 individuals with HIV-1 as of the end of 1989 were estimated, 80 per cent of adult cases occurring among IVDUs. The model included two main subgroups: IVDUs and non-drug-using heterosexuals, both with behavioural heterogeneities. Sharing of needles among IVDUs and heterosexual contact were considered as possible ways of transmitting HIV-1. A mathematical framework was developed to reproduce different mixing patterns within and between subgroups. A scenario analysis of the model showed incident cases of HIV-1 among IVDUs peaking early, then declining dramatically and stabilising at low values, with a stable-state prevalence higher than 0.75. Heterosexual interaction with IVDUs resulted in a significant, but non-self-sustaining, virus spread in the general population, affecting females more than males. The extent of this spread is associated with the assortativeness of the sexual mixing pattern adopted. The qualitative features of the Italian epidemic are well represented by the model, which highlights the role of IVDUs as an infection reservoir. However, the need emerges for more accurate information on the key parameters influencing the transmission dynamics of HIV-1.