Background: Septic arthritis is an emergency in orthopedics. Several mechanisms have been described: hematogenous spread, extension from an adjacent focus and direct inoculation, being the first one the most frequent mechanism. If not handled properly neither early, what can lead to the destruction of the articular cartilage and the production of sequelae. In an incipient case, good results can be got with conservative treatment, but usually surgical management is necessary for the resolution of the process. Material and Methods: We present a case of a 5 year old child who suffered an episode of arthritis after the administration of vaccine against serogroup B Neisseria Meningitidis. Symptoms began 2 hours after vaccination with pain and fever of up to 39ºC. At this moment, arthrocentesis was performed and intravenous antimicrobial therapy was initiated. Results: The evolution was favorable with no need of surgical treatment. Symptoms continued going down after the management with arthrocentesis and intravenous antibiotic treatment, until disappearing completely in a few days. During the follow-up, no signs of recurrence have appeared after 12 months. Conclusions: despite the fact that the hematogenous spread is the most frequent mechanism of establishment of arthritis septic, direct inoculation can justify a case of arthritis after vaccination in the deltoid region. We must be careful during vaccination with bacteria toxoids in the deltoid region, due to the risk of reactive arthritis and its differential diagnosis with septic arthritis. More studies are needed to clarify the diagnoses in the borderline cases, being molecular biology techniques as protein chain reaction a fast and useful tool.