Mitogen response of peripheral blood lymphocytes with phytohemagglutinin (PHA), conconavalin A (Con A), pokeweed mitogen (PWM), and purified protein derivative (PPD) was studied in 32 patients with psoriatic arthritis (PsA). Results were compared with those obtained from a control group with rheumatoid arthritis (RA) and another group of normal subjects. The PHA, Con A, and PWM responses were depressed in the PsA group. The degree of depression of the mitogen response was comparable to that observed in the group of patients with active RA. Sequential studies done in a small group of PsA patients revealed that the mitogen response paralleled disease activity. Improvement of disease activity was followed by increased mitogen response.