The features of Helicobacter pylori antibiotic resistance in Lisbon from 1990 to 1999 were studied. Overall resistance rates to amoxycillin, tetracycline, metronidazole, clarithromycin and ciprofloxacin were 0, 0, 30.6, 19.0 and 9.6%, respectively. The incidence of resistance to clarithromycin was much higher in isolates from children (44.8%) than adults (14.6%). For metronidazole, the contrary was observed (children: 19.0%, adults: 32.3%). Ciprofloxacin-resistant isolates were all from adult patients. Concerning the adult population, the resistance rate to metronidazole showed a slight increase during the decade, while for clarithromycin and ciprofloxacin a significant increase was observed (4.6 to 22.0% and 0 to 20.9%, respectively).