The effect of rapid weight reduction on left ventricular function and blood pressure was studied in 34 obese subjects, who all weighed more than 30% above their ideal body weight. Subjects with co-existing hypertension (N = 15) and proven coronary artery disease (N = 9) were included to assess contributions from these factors. Blood pressure (BP) was measured both indirectly and by direct ambulatory intra-arterial methods. Radionuclide ventriculography was performed at rest and at exercise, before and after dieting, which was supervised in hospital (daily intake 330 kCal day-1 for 4 weeks). Dieting induced a weight loss of (mean +/- SD) 9.6 +/- 3.5 kg (P less than 0.0001) in the whole group. The mean daytime intra-arterial blood pressure fell from 157 +/- 23/90 +/- 15 to 144 +/- 21/85 +/- 13 mmHg (P less than 0.0001). The fall in ambulatory intra-arterial diastolic blood pressure did not reach significance in the normotensive and ischaemic groups. The mean resting left ventricular ejection fraction (LVEF) in the whole group showed a significant reduction after weight loss from 61 +/- 10% to 56 +/- 6% (P less than 0.005). Although the individual groups showed a fall in LVEF, the effect was most marked in the hypertensive group, from 65 +/- 9% to 57 +/- 5% (P less than 0.002). Before dieting none of the groups achieved a 'normal' 5% rise in LVEF above basal in response to exercise, the hypertensive and the ischaemic groups both showing non-significant falls. This phenomenon was, however, reversed after weight loss.(ABSTRACT TRUNCATED AT 250 WORDS)