Recurrent and serious otitis media, and 2 Streptococcus pneumoniae bacteraemia episodes evoked an immune system deficiency in a 6-year-old girl. Upon investigation of the complement system, CH50 activity was moderately reduced and C4 antigen level was normal contrasting with low C3 antigen level. Factor 1 was undetectable. Factor I deficiency is rare, and its diagnosis has important practical consequences. Thanks to preventive antibiotic therapy with penicillin V and vaccinations against Neisseria meningitidis and S. pneumoniae, life expectancy and quality of this child can be significantly improved.