We reported previously that consumption of one cup of milk (240 mL) per day produced negligible symptoms in lactase-nonpersistent (LNP) individuals self-described as being severely lactose intolerant. We hypothesized that such LNP individuals could also tolerate two cups of milk per day if taken in two widely divided doses with food, and that psychologic factors play a role in perceptions of lactose intolerance. The Minnesota Multiphasic Personality Inventory 2 (MMPI-2) was administered to 19 LNP subjects self-described as markedly lactose intolerant (S-LNP), 13 LNP subjects who denied lactose intolerance (A-LNP), and 10 lactase-persistent individuals who believed they were lactose intolerant (S-LP). Symptoms were recorded when LNP subjects ingested 240 mL regular or lactose-hydrolyzed milk twice daily for 7 d in a double-blind crossover study. The results showed that neither LNP group had a significant increase in symptoms (P < 0.05) during the regular compared with the lactose-hydrolyzed milk periods. However, S-LNP subjects reported significantly greater gaseous symptoms than did the A-LNP subjects during both treatment periods. The MMPI-2 showed a high score on the "lie" validity scale for S-LNP subjects. We conclude that LNP subjects tolerate two cups of milk per day without appreciable symptoms. S-LNP subjects have underlying flatulence that is misattributed to lactose intolerance. MMPI-2 results were of questionable validity because of the high rate of dissimulation by LNP subjects.