Vitamin D is beneficial in patients with immune-mediated rheumatic diseases as it has been shown that it lowers the incidence risk and the level of inflammation. To examine the association between clinical outcomes and initial 25-hydroxyvitamin D [25(OH)D] concentrations in patients with the immune-mediated rheumatic diseases treated with infliximab for 9 months. This study was performed in patients with rheumatoid arthritis (RA), ankylosing spondylitis (AS) and psoriatic arthritis (PsA) treated with infliximab for at least 38 weeks. Disease activity was assessed using Disease Activity Score (DAS28) for RA and PsA and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) for AS, while the global assessment was performed using the Visual Analogue Scale (VAS). Patients were divided into 2 groups according to 25(OH)D concentration which was classified as deficient or non-deficient (below and above 50 nmol/L, respectively). Concentrations of infliximab (IFX) and C-reactive protein (CRP) were measured according to the manufacturer's instructions.This study was performed in patients with rheumatoid arthritis (RA), ankylosing spondylitis (AS) and psoriatic arthritis (PsA) treated with infliximab for at least 38 weeks. Disease activity was assessed using Disease Activity Score (DAS28) for RA and PsA and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) for AS, while the global assessment was performed using the Visual Analogue Scale (VAS). Patients were divided into 2 groups according to 25(OH)D concentration which was classified as deficient or non-deficient (below and above 50 nmol/L, respectively). Concentrations of infliximab (IFX) and C-reactive protein (CRP) were measured according to the manufacturer's instructions. The study included 23 patients (14 with RA, 6 with AS and 3 with PsA), median age 54 years, 15 females. Vitamin D deficient and non-deficient groups had median initial concentrations of 38 and 61 nmol/L, respectively. DAS28 and pain on VAS calculated at the 2nd and 38th week showed a statistically significant decrease only in RA and PsA patients with vitamin D deficiency (P = 0.02 and 0.06, respectively). Lower initial concentration of 25(OH)D in patients treated with infliximab was associated with better improvement of clinical measures (DAS28 and VAS) of disease after 9 months of therapy.