BACKGROUND: We examined the role of social networks and marijuana smoking in a community outbreak of infections due to Neisseria meningitidis. METHODS: We interviewed all patients and their contacts. Isolates were tested by pulsed field electrophoresis and multilocus enzyme electrophoresis. RESULTS: Nine cases of meningococcal disease occurred in the outbreak; isolates from seven cases with positive cultures were identical. Multiple overlapping social networks were found for case-patients and their contacts. All case-patients were linked by the marijuana-related activities of their contacts. CONCLUSION: Investigation of social networks and marijuana exposure might help identify close contacts of patients with meningococcal disease and help prevent secondary infections.