The relative effectiveness of five milk products with various levels of lactose reduction [0%, 50%, 80% (#1), 80% (#2), and 95%] was evaluated in six subjects with lactose malabsorption. Breath hydrogen was measured for 4 h after consumption of 300 mL of each product in a single-blind, randomized design. The mean +/- SEM maximum breath-hydrogen rise (ppm) after the 0%, 50%, 80% (#1), 80% (#2), and 95% lactose-reduced (LR) milks was 31 +/- 6, 7 +/- 3, 5 +/- 3, 5 +/- 2, and 8 +/- 3, respectively. The difference between whole milk and the LR milks was statistically significant (P less than 0.05) but there was no difference between any of the LR milks. Whole milk provoked symptoms in most subjects whereas 95% LR milk produced none. Only one of six subjects reacted to the 50% and 80% LR milks. The results suggest that a 50% level of lactose reduction in milk may be adequate to relieve the signs and symptoms of milk intolerance in the majority of healthy adults with lactose malabsorption.