Lactose digestion from and tolerance to flavored and frozen yogurts, ice cream, and ice milk were evaluated (20 g lactose/meal) in lactase-deficient subjects by use of breath hydrogen techniques. Unflavored yogurt caused significantly less hydrogen production than milk (37 vs 185 delta ppm X h, n = 9). Flavored yogurt was intermediate (77 delta ppm X h). Subjects were free of symptoms after consuming flavored and unflavored yogurts. Of seven commercial yogurts tested, all contained significant levels of microbial beta-galactosidase (beta-gal). In addition, eight subjects were fed meals of milk, ice milk, ice cream, and frozen yogurts with and without cultures containing high levels of beta-gal. Peak hydrogen excretion after consumption of frozen yogurt with high beta-gal was less than one-half of that observed after the other five test meals and intolerance symptoms were absent. Tolerance to frozen yogurt, produced under usual commercial procedures, was found to be similar to that of ice milk and ice cream.