Flaky skin (fsn) is an autosomal recessive mutation on mouse chromosome 17 that causes severe anaemia, forestomach papillomatosis and a papulosquamous skin disease that resembles psoriasis in humans. In the present paper, it is reported that fsn causes peripheral lymphadenopathy, CD4/CD8 imbalance and hyperresponsiveness to T cell growth factors. Peripheral lymph nodes (PLN) of adult mutant (fsn/fsn) mice were found to contain almost 10-fold more leucocytes than PLN from phenotypically normal littermates (+/fsn or +/+, hereafter referred to as +/?). Analysis of PLN cells using mAbs and flow cytometry revealed that this predominantly lymphoid hyperplasia was characterized by approximately equivalent increases in the numbers of CD3+ T cells and CD19+ B cells. However, expansion within the T cell compartment was non-random, because fsn/fsn PLN had a considerably reduced ratio of CD4+ to CD8+ T cells (1.08 +/- 0.37) compared to +/? PLN (2.47 +/- 0.44, P < 0.0001). In vitro assays of cellular proliferation in response to T and B cell growth factors showed that fsn/fsn PLN cells were hyperresponsive to IL-2, IL-4 and IL-7 when compared with PLN cells from +/? mice. Studies using mesenteric lymph node and peripheral blood cells showed that hyperresponsive cells are widely distributed in fsn/fsn mice. Experiments in newborn mice showed that the lymphoid disturbances caused by fsn are established at least as early as 2 weeks of age, a time that precedes the onset of the earliest clinical skin lesions. These data implicate a role for the fsn gene product in regulating the size and content of the peripheral lymphoid compartment.