Invasive procedures, such as the lumbar puncture, can cause anxiety and pain in children undergoing treatment for acute lymphoblastic leukemia (ALL). We investigated the safety and efficacy of two different protocols for pain relief in 20 children with ALL undergoing lumbar puncture. Protocol A was composed of an association between propofol and alfentanil. Protocol B consisted in the combination of propofol and ketamine. Vital and behavioural parameters, sedation and pain scores were recorded at different times during and after the procedure. All patients showed a satisfactory sedation and analgesia. We found a statistically significant difference of vital parameters between protocol A and protocol B, while there were no significative differences between sedation scores and the other parameters evaluated. Patients in protocol A showed a higher incidence of major side effects, such as respiratory depression. Our results show that both protocols are effective to obtain a good sedation and analgesia in children with ALL undergoing lumbar puncture, but the association between propofol and ketamine appears more safe due to the lower incidence of side effects.