To estimate the cost of the first combination antiretroviral drug therapy (cART) in HIV-infected patients and to determine factors associated with expensive prescriptions, 1698 patients starting cART between September 2002 and September 2007 were selected from the Dat'AIDS cohort. A multivariate linear regression model was used to assess associations between the cost of first cART and patient characteristics, clinical centre and cART adequacy. At cART initiation, the median age was 39 years, median CD4 count was 223 cells/mm(3), median viral load (VL) was 5.2 log copies/mL and 18.3% presented with AIDS. cART was concordant with the French guidelines in 88.7%. The mean cost of cART varied from €26.69/day/person in 2002-2003 to €32.23 in 2006-2007 (P < 0.0001), cost was associated with previous AIDS diagnosis (€31.83/day/person) versus (29.49; P < 0.0001), baseline VL > 5 log copies/mL (€30.99/day/person) versus (28.33; P < 0.0001) and centre. cART regimen not concordant with guidelines were more expensive (€38.31/day/person) versus (29.07; P < 0.0001). After adjusting for the year of initiation, the previous AIDS diagnosis, VL and recommended cART regimen, differences were still found between centres (from €27.81/day/person) to (33.12; P < 0.0001). Cost should be considered when choosing a first cART regimen, especially when considering clinically equivalent regimens.