Maximal respiratory pressures at the mouth (PEmax and PImax) have been measured in 370 normal caucasian children and adults. Age, height, and weight were recorded for all subjects and incorporated in a stepwise multiple regression analysis to determine prediction equations for the maximal respiratory pressures in the children and adults for both sexes. In men PImax and PEmax were significantly correlated only with age (p less than 0.001 and less than 0.035 respectively), whereas in women they were correlated with height (p less than 0.035 and less than 0.03). In both boys and girls PImax was related to weight (p less than 0.0001 and less than 0.01 respectively) and PEmax to age (p less than 0.001 for both). The values for PImax and PEmax in adults were lower than in previously reported series, but in children the values obtained were similar to those reported for several smaller series.