In assisted reproductive technology (ART) programmes, approximately 10% of infertile patients have at least two or three repeated implantation failures (RIFs) after an in vitro fertilization (IVF) protocol. Successful implantation mainly depends on local immune tolerance mechanisms involving a spectrum of cytokines, interleukins and growth factors. The latter have played pivotal roles in the recruitment of immune cells (and notably T-lymphocyte cells). In total, 250 couples participating in frozen–thawed embryo transfer programme were incorporated in a randomized clinical trial (peripheral blood mononuclear cells (PBMC) subgroup: n=122; control subgroup: n=128). In the PBMC group, a blood sample was collected 5 days before the scheduled frozen–thawed embryo transfer; PBMCs were isolated using Ficoll separation and then cultured for 72 h. Two days prior to embryo transfer, 0.4 ml of cultured PBMCs were transferred into the patient’s uterus. Although the clinical pregnancy rate was higher in the PBMC group (34.4%) than in the control group (23.4%), this difference was not statistically significant (P=0.05 in a chi-squared test). Nevertheless, when we limited the analysis to patients with ≥3 RIFs (n=138), there was a significant difference in the clinical pregnancy rate between the PBMC group (38.6%) and the control group (19.7%; P=0.01). Our results imply that PBMC transfer can be part of effective fertility treatment for patients with RIF.