This study was carried out to investigate serum calcium levels and other prognostic factors of patients with ATLL as well as epidemiological and clinical features of the patients. In this retrospective cohortstudy, clinical and laboratory records of 88 patients diagnosed with ATLL were reviewed. Patients were admitted to Ghaem & Imam Reza & Razavi hospital, Mashhad, Khorasan, Iran from March 21th 2005to June 21th 2017. 88 patients (49 men and 39 women) who diagnosed with ATLL were eligible for analysis. Of those 32, 31, and three patients were diagnosed with acute, lymphoma, and chronic typesof ATLL, respectively. The mean age of patients was 56.02 ± 12.724 years. Considering clinical pattern, 67 (82.7%), 20 (62.5%), 28 (42.4%),5 (11.1%), 30 (44.8%), 33 (42.9%) and 76 (98.7%) of the patientssuffered from lymphadenopathy, hepatosplenomegaly, respiratory involvement, CNS involvement, GI involvement, skin lesion and B symptom, respectively. Sixty- five patients with aggressive ATLL(lymphoma and acute type) received treatment. Among those with acute type 25 (28.4%), 4 (4.5%), 4 (4.5%) patients received first-line therapy IFN/ZDV, hyper CVAD regimen and lenalidomide/ZDV, respectively. All patients with lymphoma type received first-line chemotherapy CHOP regimen.The overall survival times (OS) was 8 months. Median survival times (MST) were 5, and 11months for acute, and lymphoma types, respectively. MST for patients with hypercalcemia was 5 months (versus 9months for patients with normal serum calcium level). MST was 5months (95% CI 3.378- 6.622) forpatients who received first-line antiviral therapy IFN/ZDV, 5 months ( 95% CI 2.06- 7.94) for patients who received first-line chemotherapy hyper CVAD regimen, and 3 months ( 95% CI 0.00- 9.86) forpatients who received first-line antiviral therapy lenalidomide/ZDV. Patients with acute ATLL, significantly benefited from first-line antiviral therapy, whereas patients with ATLL lymphoma experienced a better outcome with chemotherapy. In logistic regression analysis; serum calcium level, higher serum lactate dehydrogenase (LDH) level, atypicall cell count, serum levels of; ALT, AST, direct bilirubin, urea, creatinine and also respiratory involvement, Ann- Arbor stage , performance status , type of ATLL and treatment were associated with poorer outcome. Hypercalcemia is associated with poorer outcome and shorter survival time of patients with ATLL.