Twenty adult (20-40-y old) and 20 elderly (> or = 65-y old) Asian-Americans subjects were evaluated for baseline lactose consumption, fecal beta-galactosidase activity, and lactose maldigestion to determine whether there were differences in lactose metabolism and tolerance between these groups. Fasted subjects consumed a challenge dose of 0.5 g lactose/kg body wt. Breath-hydrogen production and symptoms were monitored. There were no statistically significant differences in total hydrogen production (P < 0.6), flatulence (P < 0.6), or fecal beta-galactosidase activity between the two groups. Fecal beta-galactosidase activity did not correlate with prior lactose consumption. The shape of the breath-hydrogen curves suggests a slightly delayed transit in the elderly subjects, but apparently this delay was insufficient to alter tolerance. Thus, the findings suggest that these two groups do not differ in their metabolism and tolerance of lactose.