Extracorporeal photochemotherapy (ECP) has demonstrated immunological effects. The proposed cytotoxic lymphocyte antigen 4 (CTLA-4) involvement, together with forkhead box P3 (FoxP3) and transforming growth factor (TGF)-β are associated with regulatory T cell activity. The aim of the study was to evaluate the regulatory T cell-associated effect of ECP in recent onset type 1 diabetic (T1D) children. Children (n = 20) with T1D received photopheresis 8-methoxypsoralen + ECP or placebo + shampheresis. Peripheral blood mononuclear cells (PBMC) collected pretreatment (day 1) and post-treatment (day 90) were stimulated with phytohaemagglutinin (PHA) and T1D-associated glutamic acid decarboxylase 65 (GAD₆₅) peptide a.a. 247–279. CTLA-4, sCTLA-4, FoxP3 and TGF-β mRNA transcription was quantified. Photopheresis-treated individuals' relative mRNA expression was generally maintained during the course of the study. Placebo individuals increased in spontaneous CTLA-4 mRNA (P < 0·05) but decreased in expression after stimulation with GAD₆₅-peptide (P < 0·05) and PHA (P < 0·05). Spontaneous TGF-β (P < 0·05) increased whereas PHA- (P < 0·01) and GAD₆₅-peptide (P < 0·01)-induced TGF-β expression decreased in the placebo group, whereas it was maintained in the treated group. Without intervention, expression of CTLA-4 and TGF-β, stimulated with PHA and GAD₆₅ peptide, decreased with time, with a parallel reduction of GAD₆₅-peptide and PHA-stimulated TGF-β expression. These parameters were counteracted by ECP. In conclusion, our results indicate that ECP maintains regulatory T cell-associated activity in recent-onset T1D.