Caloric restriction (CR) is considered to increase lifespan and to prevent various age-related diseases in different non-human organisms. Only a limited number of CR studies have been performed in humans, and results put CR as a beneficial tool to decrease risk factors in several age-related diseases. The question remains at what age CR should be implemented to be most effective with respect to healthy aging. The aim of our study was to elucidate the role of age in the transcriptional response to a 30% CR diet in immune cells, as immune response is affected during aging. Ten healthy young men, aged 20-34, and nine healthy old men, aged 64-85, were subjected to a two week weight maintenance diet, followed by three weeks of 30% CR. Before and after 30% CR, peripheral blood mononuclear cells’ (PBMCs) whole genome gene expression was assessed. Expression of 554 genes showed a different response between young and old men upon CR. Gene set enrichment analysis revealed a downregulation of gene sets involved in immune response in young men, but not in old men. At baseline, immune response-related genes were already higher expressed in old compared to young men. Upstream regulator analyses revealed that most potential regulators were controlling immune response, and were inhibited in young men upon CR, and activated in old men at baseline. Based on the gene expression data, we conclude that a short period of CR is more effective in young men compared to old men regarding immune related pathways.