OBJECTIVE We investigated whether HIV-1 infected patients receiving highly active antiretroviral therapy (HAART) and HIV-1 infected patients who had never received HAART had differences in their vascular microcirculatory function. METHODS We assessed the forearm blood flow before and after four minutes of ischemic occlusion of the brachial artery using venous occlusion strain gauge plethysmography. The hyperaemic forearm blood flow was recorded for three minutes at 15 second intervals. We calculated the maximal percent increase of the forearm blood flow during hyperemia. Forty HIV-infected male patients receiving HAART were compared to 20 age- and BMI- matched, male HIV-infected patients who had never received HAART (control group). RESULTS Patients on HAART had similar baseline forearm blood flow but lower maximal and percentage (%) change in forearm blood flow than control patients (4.2 +/- 1.7 vs. 4.1 +/- 1.7 l/ 100mL/min P = 0.8, 32 +/- 11.2 vs. 38.9 +/- 10.5 l/100 mL/min. P = 0.04 and 714 +/- 255 vs. 907 +/- 325%, P = 0.01, respectively). Patients receiving HAART had higher cholesterol than control patients (221 +/- 58 vs. 163 +/- 38 mg/dL, P = 0.001). HAART was associated with the percentage change in the blood flow during hyperemia (coefficient regression B = -0.32, P = 0.02) after adjustment for age, cholesterol and viral load. CONCLUSIONS HIV-infected patients receiving HAART present abnormalities of arterial microcirculation in comparison with never-treated patients.