Abstract Background There is little information about vitamin D (Vit D) deficiency in patients with pulmonary hypertension (PH). The objective of this study was: 1) compare Vit D levels between patients with PH, left ventricular failure (LVF) and healthy subjects (HS); 2) correlate, in patients with PH, Vit D levels with prognosis-related variables, such as the 6-min walk test (6MWT). Methods Vitamin D levels were measured in a cross-sectional study in 126 patients from one of three groups: patients with PH (n = 53), patients with LVF (n = 42) and healthy subjects (n = 31). In all groups, 8-h fasting blood samples were obtained in the morning. In the PH and the LVF group, functional class (WHO criteria), metres covered in the 6MWT and echocardiographic parameters were analysed. In the PH group, plasma N terminal pro B type natriuretic peptide (NT-proBNP) level was analysed and a complete haemodynamic evaluation by right heart catheterisation was made. Results Mean Vit D levels were lower in PH than in both other groups (ng/ml, mean ± SD): PH 19.25 ± 10, LVF 25.68 ± 12, HS 28.8 ± 12 (PH vs LVF p = 0.017, PH vs HS p = 0.001 and HS vs LVF p = 0.46). Vit D deficiency prevalence was higher in PH as compared to the other groups (PH 53.8%, LVF 45.2%, HS 25%, p = 0.01). Patients with PH in functional class (FC; WHO criteria) III–IV had higher Vit D deficiency prevalence than those in FC I–II (86.7% vs 40.5%, p = 0.003). There was a significant linear correlation between the 6MWT and Vit D levels in PH (p < 0.01), but not in LVF (p = 0.69). Conclusions Vit D levels were lower in patients with PH as compared to patients with LVF and HS and correlated directly with 6-min walk distance.