In order to evaluate factors that may influence H2 generation, the effects of age, sex, race, and functional complaints on breath H2 were studied in 62 subjects. These included 34 young hospital employees, 17 healthy ambulatory elderly subjects, and 11 patients with functional bowel disease. The ability to produce H2 was defined by an increase in breath H2 to greater than 20 parts per million within 4 hr of ingesting 10 g of the nonabsorbable sugar lactulose. Overall, 21% of subjects were nonproducers, and the incidence of nonproduction did not vary among the groups. The elderly subjects had significantly greater breath H2 concentrations than the younger subjects at 150 minutes (P less than 0.05). Sex, race, and functional complaints did not influence H2 production. These findings indicate that patient's age and potential inability to make H2 need to be considered in the routine interpretation of H2 breath tests.