Vitamin D supplementation decreases the frequency of CD4+ T cells expressing CD279 [programmed death 1 (PD‐1)] in a dose‐dependent manner. At baseline, free‐serum 25‐hydroxyvitamin D (s25OHD) was correlated negatively with the expression of human leucocyte antigen D‐related (HLA‐DR) by CD4+ T cells (a) and with the frequency of CD4+ T cells co‐expressing CD38 and HLA‐DR (b). At the end of the study, the combined group of patients receiving vitamin D2 or D3 experienced a decreased frequency of CD4+ T cells expressing CD279 (PD‐1) (c). At this time‐point, total vitamin D dose received was correlated negatively with change in CD279 (PD‐1) expression by CD4+ T cells (d) and correlated positively with change in CD4/CD8 ratio (e). P < 0·05 in all. Results are plotted as raw data from all subjects participating in the study irrespective of treatment arm (a,b,d,e) and as raw data from the subjects in the pooled vitamin D2 and vitamin D3 treatment group (c). The number of patients presented in the individual graphs varies due to a few missing samples (see Table 3).