The antibacterial effect of aqueous garlic extract (AGE) was investigated against Helicobacter pylori. Sixteen clinical isolates and three reference strains of H. pylori were studied. Two different varieties of garlic were used. The concentration of AGE required to inhibit the bacterial growth was between 2-5 mg ml-1. The concentration, for both AGE types, to inhibit 90% (MIC90) of isolates was 5 mg ml-1. The minimum bactericidal concentration (MBC) was usually equal to, or two-fold higher than, minimum inhibitory concentration (MIC). Heat treatment of extracts reduced the inhibitory or bactericidal activity against H. pylori; the boiled garlic extract showed a loss of efficacy from two- to four-fold the values of MIC and the MBC obtained with fresh AGE. The antibacterial activity of garlic was also studied after combination with a proton pump-inhibitor (omeprazole) in a ratio of 250:1. A synergistic effect was found in 47% of strains studied; an antagonistic effect was not observed.