An outbreak caused by amoxicillin-resistant Haemophilus influenzae type b was noted among patients suffering from chronic obstructive pulmonary disease. Since infections were clustered in time and place, an ongoing outbreak was suspected. The spread of the strain and the course of the outbreak could be followed by random amplification of polymorphic DNA (RAPD) analysis of the different bacterial isolates. In addition, studies were aimed at the determination of length polymorphism in regions of repetitive DNA. By PCR-mediated amplification of variable number of tandem repeat regions (VNTRs), additional insight into the genome composition of the epidemic strain was gained. Our results show that VNTRs comprising repeat units that are 3, 5, or 6 nucleotides in length provided stable genetic markers that can be used for molecular typing of H. influenzae type b. VNTRs built from tetranucleotide units, however, appear to be hypervariable and not suited for epidemiological studies. The observed variability in this latter class of VNTRs might be reminiscent of the bacterium's capacity to deal with unfavorable host factors.