The aim of this survey was to determine the vascular disturbances occurring during transient synovitis (observation hip, "hanche irritable"). Within these last two years thirty children were admitted with painful hip, limp and restriction of the passive motion in the hip joint. Legg-Calve-Perthes disease was excluded by roentgenographic analysis and bone scintigraphy. Osteoblastosis was studied by using 99mTc methyl-diphosphonate. Another tracer was necessary to investigate the blood supply into the affected femoral head : 99mTc marked colloids were injected. Immediately after the intravenous administration of the isotope comparative activity was recorded every ten seconds. Computerized information allowed the description of four different stages, i.e. arterial supply, filling peak, venous elimination and later recirculation. Nineteen (63%) affected children presented a decrease in isotopic activity during the first two stages. The proposed interpretation consists in a selective decrease in blood supply in the femoral head. Venous circulation was rarely disturbed. Treatment consisted in bedrest with skin traction. In this series, no aspiration of the joint was performed. Eight days after admission dynamic scintigraphy with marked colloids revealed a highly significant decreased asymmetry between the two femoral heads. Only seven patients (23%) presented arterial hypoperfusion. Fifteen days later only three cases remained abnormal. Two of them presented some weeks after the regression of the symptoms an asymmetrical bone scintigraphy and secondarily the roentgenographic features of Legg-Calve-Perthes disease. Radioactive colloids seem to be useful to diagnose transient synovitis and to determine the prognosis. Although its etiology is still unknown, transient synovitis seems to be characterized by vascular disturbances consisting in a decrease in blood supply in the affected femoral head. Persistence or recurrence of abnormal colloid scintigraphy is indicative of a poor prognosis.