In order to monitor the distribution of subtypes of Campylobacter and to identify clusters, 975 isolates of Campylobacter spp., obtained from human infections occurring in two Danish counties, were studied during a 1-year period. The isolates were characterised by Penner serotyping and automated ribotyping. Pulsed-field gel electrophoresis (PFGE) profiling was used to confirm clustering of identical serotypes and ribotypes. The 975 isolates were divided into 48 serotypes, 210 ribotypes and 277 serotype-ribotype combinations. The overall distribution of serotypes and ribotypes was similar between the two counties. After taking into account the rare or common occurrence of subtypes, a model identified 43 clusters of subtypes during the study period. Clustered isolates represented 28% (273/975) of the study population, with clusters containing between three and 20 isolates. PFGE confirmed the validity of selected clusters identified by serotyping and ribotyping. The observed clustering of Campylobacter isolates, with identical types in time and place, indicates that common-source outbreaks of campylobacteriosis are more common than is usually thought.