Lymphopenia is a recognized but poorly studied feature of rheumatoid arthritis (RA). We set out to establish the prevalence and significance of lymphopenia in RA. A group of 66 RA patients was studied for one year. During this time 10 (15%) had persistent lymphopenia (lymphocyte count less than 1.00 X 10(9)/l) without evidence of Felty's syndrome. A separate study of lymphocyte subsets in 13 lymphopenic RA patients showed marked reduction in T-cell numbers with normal circulating B-cell numbers. The numbers of CD4 and CD8 positive T-cells were equally depressed. Lymphopenia may indicate more severe disease. It was not influenced by changes in disease activity or therapy.