Langerhans' cell histiocytosis (LCH) is a disease with an unpredictable course and unpredictable reactivations. Since active disease may cause life-long sequelae, early detection of reactivation is important. The usefulness of erythrocyte sedimentation rate (ESR) and thrombocytosis as indicators of disease activity in patients with LCH were examined. The mean values for ESR and platelet count for each quarter of a year were compiled. The means of all the ESR estimates (n = 76) during each quarter of a year with active disease was 21 mm h(-1), with intermediate disease activity 11 mm h(-1) and with resolution 7 mm h(-1). The corresponding platelet count estimates (n = 139) were 433 x 10(9) l(-1), 365 x 10(9) l(-1) and 304 x 10(9) l(-1), respectively. In conclusion, elevated ESR and platelet count may be clinically valuable indicators of disease activity in LCH.