To assess the frequency and severity of myelosuppression due to cranio-spinal irradiation to identify patients at high risk of haematological toxicity who may require supportive therapy. Patients and Methods: Between 1998 and 2002, 45 patients received craniospinal axis radiotherapy as part of the treatment of primary CNS tumours at departement of radiotherapeutic oncology of cancer Institute. The dose to the whole brain ranged from 30 to 40 Gy in 1.5-1.85 Gy/f. The usual dose to the posterior fossa or to the site of primary disease was 50– 55 Gy in 6–8 weeks for adults and was reduced to 45–50 Gy in 6–7 weeks for children aged, 3 years. Complete blood counts (CBC) were obtained during radiation therapy. Any Interruption in treatment due hematologic toxicity (G3-4) lasting 2 days was defined as a significant treatment Interruption. Results: In 45 patients who received craniospinal irradiation (CSI) 19 patients had treatment Interruption more than 2 days. The median dose at Interruption was 21.6± 8.7 Gy and the the frequency of grade 3,4 hematologic toxicity occurred with peack incidence in fourth week of CSI(median day of Interruption was day 23,range 8-38) .The median WBC count was 1900(range 100-2140).The mean of Interruption period was 7±8 days (one patient died from neutropenic fever then he didn’t finish his treatment. In 19 patients who had treatment Interruption ,13 of them were under 12 years while 5 patients were greater than 12 years .With Mann-Whitney U test the median age of patients with treatment Interruption was lower; 10.2 years and 16.2 years respectively(P=.059). ).In 24 patients with dose/fraction =<1.6 Gy 12 patients (50%) had treatment Interruption but in 21 patients with dose/fraction >1.6 Gy 7 (33%) had treatment Interruption(P=.2) but Interruption period was longer in patients with dose/ fraction >1.6 Gy .From 12 patients who had dose/fraction <=1.6Gy ,7 patients had Interruption period less than 5 days while in group with dose/fraction >1.6 Gy all patients had treatment Interruption period more than 5 days(P=.016,CI 95% for Interruption period difference =1.2-4.7). Conclusion: .In summary, in our study 42% of patients undergoing CSI had treatment Interruption. The risk was higher in children and in patients who received higher spinal dose fraction the treatment Interruption was longer, but the overall treatment-related morbidity was low. In a population at risk of hematological toxicity where further studies of HGFs should be targeted.