Erythema nodosum is a relatively common form of panniculitis in children. The diagnosis is clinical, based upon the presence of painful erithematous nodules or plaques. Tuberculosis should be always suspected (“one”) but other infective or immunological conditions are now more frequently diagnosed (“one hundred thousand”). In most cases the cause of erythema nodosum will remain unidentified (“no one”). Therapy is that of the underlying disease, while NSAIDs and steroids can be used as symptomatic drugs for more severe cases. Erythema nodosum usually resolves in 2-6 weeks without sequelae, but relapses are possible. This short review discusses the diagnostic and therapeutic approach to erythema nodosum starting with the description of four recent cases.