Two co-primary cases of schoolchildren with acute meningococcal disease due to infection with Neisseria meningitidis serogroup B are described. The first patient presented with septic shock and purpura fulminans, bacteremia greater than 10(5) organisms/ml blood, low C3 and factor B levels and an elevated level of C3d. The second patient had meningitis, negative blood cultures, normal levels of serum complement components but more than 10(4) organisms/ml in CSF. A comparison of the quantitative bacteriology data and complement profiles for these patients suggests that the degree of activation of complement during meningococcemia is directly related to the number of organisms in the blood. This association may determine the different clinical syndromes.