Seasonal variation has been shown in meningococcal disease (MD), but it is not known whether seasonal variation depends on age, gender and meningococcal phenotype. Based on complete registration of MD in North Jutland County, Denmark, during 1980-1999 (n = 413 cases), a Poisson regression model of a sinusoidal form was used to examine: (i) whether the seasonal variation in MD follows a sinusoidal pattern; (ii) the magnitude of seasonal variation in MD; and (iii) whether seasonal variation is related to age and gender and meningococcal phenotype. The peak-to-trough ratio (PTR) was used as a measure of the magnitude of seasonal variation. An overall seasonal variation in MD was found [PTR 2.0, 95% confidence interval (95% CI) 1.5-2.6]. There were differences between age groups and between phenotypes, but not between genders. The highest PTRs were observed for the age groups 5-9 y (PTR 4.9, 95% CI 2.1-11.9) and 10-14 y (PTR 3.7, 95% CI 1.4-9.7). No seasonal variation was found for children < 1 y (PTR 1.4, 95% CI 0.6-3.2) or teenagers 15-19 y (PTR 1.6, 95% CI 0.8-3.0). Among phenotypes the highest PTR was observed for C:2a:P1.2,5 (PTR 3.8, 95% CI 1.3-11.2). Thus, seasonal variation depended on age and meningococcal phenotype, the most pronounced seasonality being in 5-14-y-olds and in cases with phenotype C:2a:P1.2,5.