Formaldehyde, acetaldehyde and acetone expired from tumor-bearing transgenic mice and formaldehyde exhaled from breast cancer patients were analyzed using gas chromatography. The tumor-bearing mice expired significantly more formaldehyde per unit metabolic size (1.43-2.98 micromol) than did control mice (0.77-1.01 micromol). There was no detectable difference in the levels of expired acetaldehyde and acetone between the two groups of mice. The exhaled formaldehyde levels from three women with breast cancer and from three healthy women were satisfactorily determined using the method developed in this study. The results suggest that these carbonyl compounds may be used as a biomarker.