Background Early identification of patients in whom left ventricular dilatation is likely to occur may have important therapeutic implications. Thus the purpose of this study was to evaluate the relation between Killip class on admission and subsequent left ventricular dilatation after acute myocardial infarction. Methods and Results We serially evaluated 129 consecutive patients by two-dimensional and Doppler echocardiography on days 1, 2, 3, and 7, at 3 and 6 weeks, and at 3, 6, and 12 months after infarction. Killip class on admission >1 was found in 29 of 129 (22.5%) patients, and they had significantly higher end-systolic and end-diastolic volume indexes and wall motion score index from day 1 onward compared with patients with Killip class 1, whereas ejection fraction was lower during the follow-up period in these patients. Patients with Killip class >1 showed the progressive increase of end-diastolic (68.2 ± 2.99 to 88.0 ± 7.55 ml/m², p = 0.001) and end-systolic volume indexes (43.9 ± 2.67 to 56.3 ± 6.18 ml/m², p = 0.004) during the follow-up period, whereas ejection fraction and wall motion score index remained unchanged. In patients with Killip class 1, end-systolic volume index did not change (30.8 ± 1.06 to 33.8 ± 2.15 ml/m², p = 0.064), ejection fraction increased (49.3% ± 0.99% to 51.8% ± 1.17%, p = 0.027), and wall motion score index decreased (1.50 ± 0.03 to 1.35 ± 0.04, p < 0.001). End-systolic volume index was the major independent correlate of Killip class, followed by history of diabetes and peak creatine kinase level. No association was found between Doppler indexes of diastolic filling and Killip class on admission. Conclusions Killip class >1 on admission is associated with both acute and long-term left ventricular dilatation. On the other hand, Killip class 1 is associated with favorable left ventricular functional indices, and it appears that left ventricular function in these patients may improve over time. Initial end-systolic volume index but not ejection fraction is the major correlate of Killip class. (Am Heart J 1999;137:361-7.)