Probiotic supplementation may provide health benefits, especially for individuals with an underlying disease state that makes them more susceptible to infections. The purpose of this experiment was to evaluate the safety and tolerance of Lactobacillus reuteri ingestion by subjects infected with the human immunodeficiency virus (HIV). Thirty-nine subjects consumed a freeze-dried preparation of L. reuteri or a placebo for 21 days in a double-masked, parallel design experiment. Serum chemistry, haematology, immune profile, urinalysis, physical examination, gastrointestinal tolerance and faecal microbiota data were collected. No clinically significant changes were noted in any of the safety parameters measured. Overall, tolerance was good in both groups. Consumption of L. reuteri tended to increase faecal levels of L. reuteri on days 7, 14 and 21 of treatment feeding (P < 0.06, P < 0.11 and P = 0.05, respectively). However, faecal levels of L. reuteri and total Lactobacillus species were lower than levels previously observed in healthy male adults. Overall, this study documents that L. reuteri may be fed to HIV-positive individuals at 1 x 10(10) colony forming units/day without any clinically significant safety or tolerance problems.