This is an open-label trial investigating the efficacy of temsirolimus in recurrent or refractory primary CNS lymphoma (PCNSL) after initial chemotherapy with a high-dose methotrexate containing regimen. 37 patients are planned to be treated with weekly infusions of 25mg Temsirolimus (first 3 patients) or 75mg Temsirolimus (all other patients) up to a maximum of 12 months. The trial is designed in two stages, if less than one of the first 12 patients responds to treatment, the trial is stopped. In addition to efficacy, safety and penetration of temsirolimus into the cerebrospinal fluid will be investigated.