The cardiovascular benefits of blood pressure lowering in obese people compared with people of normal weight might depend on choice of drug. We compared the effects of blood pressure-lowering regimens on cardiovascular risk in groups of patients categorised by baseline body-mass index (BMI). We used individual patient data from trials included in the Blood Pressure Lowering Treatment Trialists' Collaboration to compare the effects of different classes of blood pressure-lowering regimens for the primary outcome of total major cardiovascular events (stroke, coronary heart disease, heart failure, and cardiovascular death). We used meta-analyses and meta-regressions to assess interactions between treatment and BMI when fitted as either a categorical variable (<25 kg/m(2), 25 to <30 kg/m(2), and ≥30 kg/m(2)) or a continuous variable. Analyses were based on 135,715 individuals from 22 trials who had 14,353 major cardiovascular events. None of the six primary comparisons showed evidence that protection varied by drug class across the three BMI groups (all p for trend >0·20). When analysed as a continuous variable, angiotensin-converting-enzyme inhibitors gave slightly greater protection for each 5 kg/m(2) higher BMI than did calcium antagonists (hazard ratio 0·93, 95% CI 0·89-0·98; p=0·004) or diuretics (0·93, 0·89-0·98; p=0·002). The meta-regressions showed no relation between BMI category and the risk reduction for a given fall in systolic blood pressure. By contrast with a previous report, we noted no relation between BMI and the efficacy of calcium antagonists compared with diuretics. We found little evidence that selection of a particular class of blood pressure-lowering drug will lead to substantially different outcomes for individuals who are obese compared with those who are lean. None.