BACKGROUND In recent years an increase has been observed in the prevalence of meningococcal infection by Neisseria meningitidis serogroup C and in the appearance of strains with moderate resistance to penicillin. METHODS A microbiologic study of the cases of meningococcal infection of serogroup C treated from 1995 to 1996 in the health care area of Ferrol (La Coruña, Spain) was carried out. CONCLUSIONS Twenty-nine cases were detected in 1995 and 28 in 1996. Meningococcal infection was observed in patients ranging from 8 months to 21 years of age (mean 5.7 years). Distribution by sex was homogeneous. Two patients died. According to the clinical presentation, 11 were sepsis (38%), 4 meningitis (14%) and 14 both processes (48%). In 4 LCR samples, the analytical study was normal with posterior positive culture results. The detection of bacterial antigen by latex agglutination in CSF only detected 32% of the cases. MIC study determined that 11 strains (38%) presented moderate resistance to penicillin, 9 with a MIC of 0.12 microgram/ml, one with a MIC of 0.25 microgram/ml and another with a MIC of 0.5 microgram/ml. In all the cases the strains were sensitive to cefotaxime (MIC < or = 0.06 microgram/ml) and rifampicin (MIC < or = 0.5 microgram/ml). All the strains belonged to serogroup C serotype 2b, serosubtype P1.2,5. During the study period 4 additional cases of meningococcal disease by serogroup B were observed.