The authors report a case of Libman-Sacks endocarditis of the posterior mitral leaflet in a child treated for disseminated lupus erythematosus for 8 years. Mitral valve replacement was performed. Libman-Sacks vegetations are hardly ever recognised in vivo and have never been previously reported in a child. The incidence of this cardiac complication of lupus erythematosus is probably higher than indicated in the literature. Attentive cardiac examination and systematic echocardiography should be part of routine follow-up of patients with disseminated lupus erythematosus.