In 1991 a multicentre survey on the prevalence of metronidazole resistance in Helicobacter pylori in vitro was carried out in 12 hospitals in 11 different European countries. The susceptibility of Helicobacter pylori to metronidazole was determined in each centre by measuring the MIC on agar with the PDM E-test (AB Biodisk) according to a standard procedure. Overall, 122 of 443 (27.5%) strains tested were resistant to metronidazole (MIC > 8 micrograms/ml). The level of resistance to metronidazole varied markedly between centres (from 7% to 49%) and was found to be substantially higher in Africans and other non-Caucasian subjects than in natives from European countries. The overall rate of resistance to metronidazole was higher in females (34.7%) than in males (23.9%), and varied according to the age group, being highest among women aged 20 to 39 (50%). Previous use of metronidazole was reported in only 16 patients, 11 of whom (68.8%) harboured resistant Helicobacter pylori strains. Although differences in the rate of metronidazole resistance in Helicobacter pylori most probably relate to variations in use of this drug in different populations, such use may frequently go unrecognized. This study emphasises the importance of monitoring the drug resistance of Helicobacter pylori on a local basis. Standardisation of the methods for testing the susceptibility of Helicobacter pylori in vitro is clearly needed for this purpose.