The impact of pregnancy on respiratory function and whether this is influenced by living conditions, such as altitude of residence, must be determined if the management of pregnant women, particularly those with pulmonary disease is to be optimized. Pulmonary diffusing capacity, corrected for haemoglobin concentration (DL(COc)), therefore, was measured in 112 healthy Peruvian women with singleton pregnancies living at sea level and 192 living at 4300 m. At each location, 19 non-pregnant women were studied. The mean DL(COc)s of the pregnant and non-pregnant women studied at sea level were similar but lower than those of their high altitude counterparts (P<0.001, P<0.001). At high altitude, the mean DL(COc)s of women studied in the first and second trimester were similar to that of non-pregnant women, but the mean DL(COc) of pregnant women studied in the third trimester was lower than that of the non-pregnant women (P<0.01). Our results demonstrate that the effect of pregnancy on pulmonary diffusing capacity is influenced by altitude of residence.