There is a close relation between T helper (Th) 1 cells and nitric oxide in disease. Thus it is possible that a reciprocal regulatory mechanism exists between them. This paper briefly describes the experimental studies which have helped elucidate the mechanism by which nitric oxide selectively enhances Th 1 cell proliferation and the potential effect of nitric oxide on regulatory T (Treg) cells. On the basis of the results the authors propose that nitric oxide represents an additional signal for the induction of T cell subset response, contributing to the increasingly complex network of immune regulation essential for health and disease.