A nationwide laboratory-based surveillance study of invasive group A streptococcal (GAS) infections was conducted in The Netherlands from May 1994 until December 2003 (average population during this period was 15 729 704). Microbiologically invasive isolates were obtained from 1504 patients, with most (70%) isolates cultured from blood. There was a clear seasonal pattern in invasive streptococcal infections, with an estimated annual incidence that peaked in 1996 (4.0 cases/100 000 individuals/year) and was at its lowest in 1999 (2.0 cases/100 000 individuals/year). Twenty-eight different M-types were identified, of which the most frequent were M1 (339/1504, 23%), M3 (187/1504, 12%), M89 (174/1504, 12%), M28 (164/1504, 11%), M12 (109/1504, 7%) and M6 (55/1504, 4%). There was a high degree of variation in the relative annual contributions of the predominant M-types, but variations in M1 and M3 combined correlated with overall changes in the annual incidence. The contribution of the patient group aged > or = 56 years to all cases of invasive GAS disease increased during the study period, whereas that of the group aged 0-20 years decreased. A peak in the incidence of invasive GAS disease among the patient group aged 30-34 years did not vary during the study period, indicating that the high incidence of invasive GAS disease in this age group was age-specific rather than cohort-related.