Vitamin D supplementation has been proposed as a potential treatment to delay amyotrophic lateral sclerosis (ALS) progression. The aims of this study were to compare retrospectively vitamin D blood levels in ALS patients with those in healthy subjects; to correlate vitamin D blood levels with clinical functions in patients; and to evaluate whether administration of vitamin D could modify the clinical progression of the disease. Vitamin D blood levels were evaluated in 57ALS patients and in 57 healthy subjects. In the ALS patients the following clinical variables were evaluated every 3 months: Medical Research Council scale (MRC) score; revised ALS functional rating scale (ALSFRS-R) score; forced vital capacity (FVC). Twentyfour patients were treated with high doses of cholecalciferol. No significant differences were found between the vitamin D blood levels in the ALS patients (18.8 ± 12.2) and the healthy subjects (20.7 ± 10.1). The vitamin D levels in the ALS patientsdid not correlate with recorded clinical parameters. No clinical differences in terms of ALSFRS-R, MRC or FVC were found between the treated and the untreated patients over time. In ALS, as in other chronic neurological diseases, levels of vitamin D in blood appeared reduced, but no difference was found between the levels in ALS patients and in healthy subjects. Oral vitamin D supplementation in ALS patients was not associated with better prognosis in comparison with untreated ALS patients. Further prospective controlled studies are needed to clarify the effect of vitamin D on the progression of ALS disease.