Alterations of blood analyses have been studied at relapse of patients with osteosarcoma and Ewing's sarcoma. The tests included erythrocyte sedimentation rate (ESR), haemoglobin (Hb), leukocyte and thrombocyte counts, gamma glutamyltransferase (GT), lactate dehydrogenase (LD) and alkaline phosphatase (ALP). Sixteen relapsing patients diagnosed from 1970 to 1987 were eligible in each sarcoma group. Median age was 16 years (range 9-30) at diagnosis. The blood tests seemed to be of no help in detecting relapse of osteosarcoma, while ESR, LD and GT rose significantly in relapsing patients with Ewing's sarcoma. ESR was best correlated to disease activity.