Scrub typhus is responsible for a large proportion of undifferentiated fevers in south-east Asia. The cellular tropism and pathophysiology of the causative agent, Orientia tsutsugamushi, remain poorly understood. We measured endothelial and leucocyte activation by soluble cell adhesion molecule enzyme-linked immunosorbent assays in 242 Lao and Thai patients with scrub or murine typhus, leptospirosis, dengue, typhoid and uncomplicated falciparum malaria on admission to hospital. Soluble E-selectin (sE-selectin) levels were lowest in dengue, sL-selectin highest in scrub typhus with a high sE-selectin to sL-selectin ratio in leptospirosis patients. In scrub typhus patients elevated sL-selectin levels correlated with the duration of skin rash (P = 0·03) and the presence of eschar (P = 0·03), elevated white blood cell (WBC) count (P = 0·007), elevated lymphocyte (P = 0·007) and neutrophil counts (P = 0·015) and elevated levels of sE-selectin correlated with the duration of illness before admission (P = 0·03), the presence of lymphadenopathy (P = 0·033) and eschar (P = 0·03), elevated WBC (P = 0·005) and neutrophil counts (P = 0·0003). In comparison, soluble selectin levels in murine typhus patients correlated only with elevated WBC counts (P = 0·03 for sE-selectin and sL-selectin). Soluble intercellular adhesion molecule-1 and soluble vascular adhesion molecule-1 levels were not associated significantly with any clinical parameters in scrub or murine typhus patients. The data presented suggest mononuclear cell activation in scrub typhus. As adhesion molecules direct leucocyte migration and induce inflammatory and immune responses, this may represent O. tsutsugamushi tropism during early dissemination, or local immune activation within the eschar.