Prediction equations for FVC, FEV1, FEV1/FVC, FEF25-75%, FEF75-85%, FEF50% and FEF75% were modelled for 396 normal non-obese adult lifetime nonsmokers and continuing smokers. Subjects came from a random cross-section of the white population of Michigan, a large industrial state. In both sexes, linear models utilizing age and height were appropriate for FVC, FEV1/FVC and FEV1; flows were better described by logarithmic transformation. These regression equations provided similar predicted values for FVC and FEV1 in nonsmokers to other models in wide use. Previous equations for instantaneous flows have yielded inconsistent predicted values. It is hoped that the present equations will be useful for these measurements. Duration of cigarette smoking was a significant prediction variable for FEV1, FEV1/FVC and mean and instantaneous flows in these normal men, but not in normal women. For FEV1, the effect of each year of smoking was 40% of the effect of ageing.