Recurrence of Helicobacter pylori infectionafter successful eradication occurs and is associatedwith relapse of gastroduodenal diseases. The aims ofthis paper were to assess the incidence and identify the nature and possible causes of recurrence ofthe infection. A broad-based Medline search wasperformed to identify all related publicationsaddressing recurrence of the infection between 1986 and1995. The 12-month recurrence rate varied among thedifferent studies from 0 to 41.5%. A few studies showed18- to 24-month recurrence rates, which ranged between0 and 21.4%. Limited data, obtained using molecular fingerprinting techniques, have shown that inmost cases recurrence is due to recrudescence of theoriginal strain; a few cases appear to be due toreinfection with a new strain. Recrudescence is mostlikely during the first 12 months after apparenteradication. Despite the high sensitivity andspecificity of the available individual tests fordetecting H. pylori infection in untreated patients, notechnique alone is sensitive enough to monitoreradication when the four-week-rule definition foreradication is used. A combination of two or moretechniques increases sensitivity. Sensitivity andspecificity are increased when biopsies are taken from bothgastric antrum and corpus. The best treatments have thelowest recurrence rates and recurrence is rare when theeradication rate is over 90%. Individual susceptibility and reexposure to H. pylori are suggested astwo major causes of reinfection.