We studied the variability and reliability of breath hydrogen and methane as well as the alterations in intestinal gas profile in response to lactulose ingestion in 13 asymptomatic male patients with diabetes mellitus (DM). Seventeen healthy subjects served as controls. The prevalence of methane producers was 33% in DM and 45% among control subjects (P = NS). The prevalence of nonhydrogen producers was 7.7% and 5.9%, respectively. The coefficient for interday variation of H2 was 72.6 +/- 9.8% in DM and 49.7 +/- 9.8% in controls (P < 0.05). Similarly, the coefficient for interday variation of CH4 was 94.3 +/- 18.8 and 69.4 +/- 16.8% respectively (P = NS). The reproducibility of basal H2 and CH4 among diabetics as assessed by r1 (measure of reliability) was poorer among diabetics when breath analysis was performed on different days (P < 0.001). There was no significant difference between diabetics and controls with respect to basal or peak or area under the curve for H2 and CH4 in response to lactulose. We conclude that there is poor reproducibility of fasting breath gas levels among asymptomatic male subjects with diabetes. In addition, DM is not associated with alterations in hydrogen- or methane-producing potential.