The aim of this study was to characterise the group B streptococci (GBS) isolates causing severe invasive infections in patients >15 years of age in Denmark from 1999 to 2004. A total of 411 invasive GBS isolates were phenotypically characterised by the capsular polysaccharide (CPS) serotype and protein Calpha, Cbeta and R4. The incidence of invasive GBS disease ranged from 2.2 to 3.2 per 100,000 adults in the study period, being highest among adults over 65 years of age. Diabetes was observed in 15% of the cases, 12% had alcohol abuse and 7% had cancer. Of all isolates, 77% were CPS serotypes Ia, Ib, III or V. The surface proteins Calpha or R4 were detected as the only protein in 57% of the GBS isolates. Cbeta was detected in 12% of the isolates, but always in combination with either Calpha or both Calpha and R4. The incidence of invasive GBS infections continued to increase in Denmark from 1999 to 2004. In that period, the overall case fatality was 14%. The most prevalent CPS serotypes were serotypes III, Ia, V and Ib. The most prevalent surface protein was R4 when testing for R4, Calpha and Cbeta. There was no clear relation between the GBS phenotype and infections with fatal outcome.