Assessment of respiratory muscle strength is done most directly by measuring maximal static inspiratory and expiratory mouth pressures (MIPS and MEPS, respectively). The available studies that report reference values of MIPS and MEPS, however, show ill-explained wide variability, not only between individuals but also between studies. This study of 106 normal white adults (60 women and 46 males, aged 16 to 79 yr) attempts to identify the anthropometric factors which best predict MIPS and MEPS. It was found that: 1) smoking does not affect MIPS and MEPS; 2) sex is a major determinant of MIPS and MEPS, as women reached 68 and 63%, respectively, of the male values; 3) within each sex, age is the major determinant of MIPS and MEPS, since body size factors such as height, weight and percent ideal body weight do not significantly improve the relationship between age and MIPS or MEPS. In both sexes, the pattern of change in pressures with age is different for MIPS and MEPS, suggesting different maturation processes for MIPS and MEPS. While MIPS is an inverse linear function of age (i.e. MIPS decreases with advancing age from early adulthood on), the relationship between MEPS and age is best described by a second degree polynomial (i.e. MEPS increases towards a peak in mid-life, after which it also decreases with age).(ABSTRACT TRUNCATED AT 250 WORDS)