Three aspects of polymorphonuclear leucocyte (PMN) function were studied in 19 patients with Behçet's syndrome (BS). By 2 different techniques directed motility was found to be increased. This increase was largely due to the subgroup of patients with ocular involvement. Counts of absolute numbers of cells migrating highlighted this finding. No difference was found in the phagocytic or adherent properties of PMN in Behçet's syndrome. Increased PMN motility in Behçet's syndrome may contribute to the expression of the syndrome. It remains to be tested whether altered PMN motility in this syndrome is genetically linked.