Blood T lymphocyte subpopulations were studied by using various monoclonal antibodies in 20 patients with ankylosing spondylitis (AS) and in 20 age and sex matched controls. A significant decrease in OKT4 (p less than 0.04) and OKT8 (p less than 0.02) lymphocytes was demonstrated in AS patients. OKT4 cells were significantly decreased (p less than 0.02) in patients with a severe ankylosis and in those with the longest disease duration. This decrease was not explained by older ages. Thus there was an negative correlation between OKT4 cell ratio and disease duration. No modification in T cell subpopulations was noted when considering HLA B27 positivity or disease activity.