OBJECTIVES: We studied the impact of changes in 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D (1,25(OH)(2)D) on regulatory T lymphocytes (Tregs) in patients with chronic pancreatitis (CP) and fat malabsorption in a prospective clinical trial. METHODS: The patients were randomized to 1 of 3 treatments during 10 weeks: weekly UV-B in a tanning bed (group A), 1520-IU/d vitamin D supplement (group B), or placebo (group C). A placebo tanning bed was used in groups B and C. We determined the levels of CD4 Tregs (CD3(+)CD4(+)CD25(+)CD127(low)FoxP3(+)) and CD8(+) Tregs (CD3(+)CD8(+)CD25(+)CD127(low)FoxP3(+)), together with 25OHD and 1,25(OH)2D. For baseline comparisons, we included 8 healthy individuals. Of the 30 included patients, 27 (group A, 7 patients; group B, 9 patients; and group C, 11 patients) completed the protocol. RESULTS: The baseline levels of CD4(+) Tregs relative to total CD4(+) count were higher in 22 patients with CP compared with healthy controls (2.8% vs 1.9%, P < 0.05) and were comparable for CD8+ Tregs (0.13% vs 0.05%, P = 0.3). Increases in levels of CD4(+) Tregs correlated to changes in 1,25(OH)(2)D (2% per 100 pmol/L, P = 0.002) and 25OHD (3% per 100 nmol/L, P = 0.01). CONCLUSIONS: Patients with CP have elevated relative levels of CD4(+) Tregs. Increases in 25OHD and 1,25(OH)(2)D were both related with increases in levels of Tregs.