The purpose of this study is to determine drug costs and the sociodemographic and clinical determinants of drug costs in a large open cohort of HIV-positive adult men and women in British Columbia, Canada from 1993 to 1995. The study is descriptive and population-based using time-series data. Individuals included in the study were men and women enrolled in a province-wide HIV/AIDS drug treatment programme who had given consent to access their entire prescription records held by the provincial drug plan. The primary outcome measure was the average annual drug cost for treating HIV/AIDS among programme participants; results were stratified by sociodemographic status, drug category and disease severity. The analysis was restricted to 1271 consenting programme participants. The average annual drug cost per participant in 1996 Canadian dollars ($Can) was $Can3431 in 1993, $Can3892 in 1994 and $Can4377 in 1995. Between 1993 and 1995, the average annual cost of antiretroviral therapies increased by 6.6% ($Can2579 in 1995), anti-infectives increased by 61.4% ($Can2402 in 1995) and other drugs used in the management of patients with HIV disease increased by 35.7% ($Can1146 in 1995). The overall increase over this period was 27.6%. In 1995, the average annual drug cost was $Can4720 for those on social assistance, $Can4545 for seniors and $Can5937 for the general population (nonpoor and nonelderly). In 1995, the average annual drug cost was $Can6887 for those with AIDS and $Can3911 for those without. Multivariate modelling indicated that drug costs were significantly related with CD4+ cell count (p < 0.001), diagnoses of AIDS (p < 0.001), provincial drug plan type (p = 0.002), time participants had spent in the HIV/AIDS Drug Treatment Program (p = 0.003), number of hospitalisations (p = 0.003) and patient's age (p = 0.004). Our data demonstrate that the average annual drug costs for persons with HIV/AIDS in British Columbia have been increasing even though the cost of antiretrovirals has been relatively stable. This was due mainly to the increase in the cost of anti-infectives and other drugs. Important cost drivers were participant's age, socioeconomic status (by provincial drug insurance coverage level) and rate of disease progression as measured by CD4+ cell counts, hospitalisation events and duration of illness.