OBJECTIVES: To investigate the consequences of longstanding obesity on left ventricular mass and structure and to examine the effects of weight loss on these variables. DESIGN: Cross sectional survey and controlled intervention study. SETTING: City of Gothenburg and surrounding areas. Sweden. SUBJECTS: 41 obese patients treated with weight reducing gastric surgery, 31 obese patients treated conventionally, and 43 non-obese subjects. MAIN OUTCOME MEASURES: Changes in left ventricular mass and relative wall thickness. RESULTS: Obese patients had higher blood pressure, greater left ventricular mass, and increased relative wall thickness than did matched non-obese control subjects. Obese subjects treated with gastric surgery had a substantial weight loss and a significant reduction in all variables when compared with conventionally treated obese subjects. Univariate and multivariate analysis of pooled data from the two groups of obese subjects showed that changes in relative wall thickness and left ventricular mass were more closely related to the change in weight than to the concomitant change in blood pressure. CONCLUSIONS: Structural heart abnormalities occurring in conjunction with obesity diminish after weight loss. The regression in these structural aberrations is better predicted by the weight loss than by the accompanying reduction in blood pressure. To prevent or improve abnormalities of heart structure in obese people, weight control should be the primary goal; it should be regarded as at least as important as regulating blood pressure.