A patient is described who after initiation of treatment with clozapine had twice a period of some days with fever, leukocytosis and elevated sedimentation rate. An infection or malignant neuroleptic syndrome could be excluded. The patient was eventually successfully treated with clozapine. Some literature concerning these adverse effects is discussed. It is important to differentiate between dangerous and transient non-dangerous adverse effects of clozapine, so that treatment is not stopped unnecessarily.