Left Ventricular (LV) remodeling after Q-wave anterior acute myocardial infarction (AMI) was examined with single-photon emission computed tomographic thallium imaging. Initial (after adenosine infusion) and 4-hour delayed reinjection images were obtained in 34 patients aged 65 ± 12 years. Short-axis slices from the delayed images were quantitatively analyzed by measuring the outer and inner diameters, and wall thickness. The results were compared with those in a group of normal subjects. The outer diameter was greater in patients than in normal subjects at the apical, mid- and basal levels (all p <0.01); the average outer diameter was 16.9 ± 1.9 mm in patients, and 12.2 ± 1.3 mm in normal subjects (p <0.001). Similarly, the inner diameter was greater in patients than in normal subjects at the 3 levels (all p <0.05); the average inner diameter was 6.5 ± 1.8 mm in patients, and 4.7 ± 1.3 mm in normal subjects (p <0.01). Wall thickness was greater in patients than in normal subjects (5.2 ± 0.5 vs 3.8 ± 0.5 mm; p <0.0001). There were significant correlations between LV dilation and time elapsed (in weeks) since AMI (r = 0.57; p <0.005), and the size of the perfusion abnormality (r = 0.44; p <0.03). Thus, LV dilation occurs after Q-wave anterior AMI, and is related to infarct size and duration. These changes can be studied by single-photon emission computed tomogmphic thallium imaging. © 1993.