We studied premature atherosclerosis with carotid Doppler ultrasonography in active acromegaly before and after treatment. Patients (n = 27) with active acromegaly and 12 age-, gender-, and body mass index-matched healthy individuals were included in the study. Carotid intima-media thickness was decreased significantly in the inactive group after treatment (median: 0.6 mm, interquartile range [IQR]: 0.55-0.80]) when compared with the active group (median: 0.9 mm [IQR: 0.75-1.15], P < .0001), but there was no significant difference between the inactive and control groups. There was a correlation between homeostasis model of assessment-insulin resistance (P = .01, r = .41) and growth hormone (GH; P < .0001, r = .46). In conclusion, premature atherosclerosis was demonstrated in active acromegaly patients probably as a consequence of insulin resistance and direct vascular effects of GH and/or insulin-like growth factor 1.