Measurements of the single-breath carbon monoxide diffusing capacity (DL) were obtained on a randomly selected sample representative of the white non-Mexican-American population of Tucson, Arizona. Methods of measurement followed the guidelines set forth in the ATS-sponsored Epidemiology Standardization Project. There were 228 healthy nonsmokers who had duplicate tests that met the criteria for being acceptable. On the basis of data from these subjects, reference equations were derived for DL, alveolar volume (VA), and DL/VA. The data demonstrate the effects of growth and development, height, and age on these variables. Because a significant proportion of women, but not of men, had low hematocrit values, an effect of hematocrit on DL and DL/VA could be demonstrated only in females in this population sample.