A hypothesis is proposed to explain the nature of the Reed-Sternberg (RS) cell and the unexpectedly common clinical associations between lymphomatoid papulosis, mycosis fungoides, and some types of Hodgkin's disease. The RS cell appears to be a lymphoblast arising from activated helper T cells. In lymphomatoid papulosis, a cutaneous eruption with histological features of Hodgkin's disease, there is a spectrum of activated helper T cells, including cerebriform cells and large transformed RS-like cells. Clonal expansion of cerebriform cells in lymphomatoid papulosis leads to mycosis fungoides. Similar expansion of large transformed cells results in Hodgkin's disease. Progressive transformation of T4-positive cerebriform cells to Ki-1-positive RS cells accounts for rare cases of coexistent mycosis fungoides and Hodgkin's disease. Confirmation of this hypothesis should focus attention on the family of human T-cell leukaemia/lymphoma viruses as possible aetiological agents in these helper-T-cell disorders.