Acute lymphatic leukemia (ALL) represents one of the most frequent malignancies in childhood. Central venous access ports or partly implanted silicone catheters are usually placed for high-dose chemotherapy in these children. We report two patients aged 7 and 3 years with acute lymphoblastic beta-cell leukemia (B-ALL), a less common subtype of ALL, which presented with hyperthermia (38.4 degrees C and 39 degrees C) during anesthesia with isoflurane for implantation of a central venous catheter. The hyperthermic reactions were accompanied by an increase in expired CO2 and acidosis as well as moderate elevation of heart rate and blood pressure. As in both patients the history and preoperative findings did not reveal signs of infection or other causes of fever, the observed alterations were interpreted as symptoms of malignant hyperthermia triggered either by succinylcholine or isoflurane, which were used in both children. In addition, the hyperthermia responded to administration of dantrolene sodium according to dose recommendations for treatment of malignant hyperthermia. In one of the patients, withdrawal of dantrolene during the initial postoperative hours was followed by a recurrent increase in body temperature, which once again could be suppressed by additional dantrolene infusion. According to the literature, malignant hyperthermia has occasionally been described in children with malignancies such as leukemia or Burkitt's lymphoma. Our observations indicate that children with B-ALL may be especially susceptible to malignant hyperthermia. Close monitoring of body temperature and expiratory CO2 are therefore indicated in these children, and dantrolene therapy should be started immediately in case of increased temperature during anesthesia.