The aim of the study was to analyze the impact of treatment with vitamin D in the progression of Alzheimer's disease. We performed a retrospective study including patients with mild stage of Alzheimer's disease with more than four years of follow-up. The presence of cardiovascular risk factors, osteoporosis, treatment with memantine, acetylcholinesterase inhibitors drugs and vitamin D were analyzed as independent variables. Time of progression to moderate and severe Alzheimer's disease was analyzed as dependent variable. The analysis was done using multivariate linear regression model, Kaplan Meier analysis, Chi-square and T test. Two hundred and two patients met the inclusion criteria. 11% of the patients (n = 23) remained in the mild stage of the disease, 54% (n = 110) developed the moderate form in a mean time of 3 ± 1.4 years while 35% (n = 69) developed the severe form in a mean time of 4.6 ± 1.4 years. Time of progression to severe stage of Alzheimer's disease was slower in patients under treatment with vitamin D compared with those without treatment (5.4 ± 0.4 years vs. 4.4 ± 0.16 years respectively, p=0.003). Treatment with vitamin D may be an independent protecting factor in the progression of Alzheimer's disease.