Objectives: T cell abnormalities have been repeatedly reported in adult patients with mood disorders, suggesting a role of these cells in the pathogenesis of these disorders. In the present study, we explored the dynamics of circulating T cell subsets over time in a population at high familial risk for developing a mood disorder. Methods: Children of a parent with bipolar disorder (bipolar offspring, N = 140) were assessed at three time-points: adolescence, young adulthood and adulthood. We carried out a detailed fluorescence activated cell sorting (FACS) analysis to determine various T cell subsets from frozen stored peripheral blood mononuclear cells of bipolar offspring and age- and gender-matched healthy controls at each time-point. Results: Throughout the period of observation reduced levels of CD3+ and CD3+ CD4+ T cells were observed. In bipolar offspring T(h)1, T(h)2, T(h)17 and natural T regulatory cells (T-regs) followed a dynamic course over time with reduced levels of T, in adolescence and a reduced relative number of T(h)1, T(h)17 cells in young adulthood. In post hoc analysis T-regs were inversely associated with the pro inflammatory monocyte state determined previously (r(s)=-0.220, p = 0.001). Significant associations between T cell subset abnormalities and psychopathology such as mood disorders were not found. Conclusions: A subtle partial T cell defect was present in bipolar offspring from adolescence through adulthood. Within this defect the dynamic change of inflammatory and regulatory T cell subsets suggests a high inflammatory state during adolescence, a reduced inflammatory state during young adulthood and a virtually normalized state at adulthood. (C) 2016 Elsevier Inc. All rights reserved.