The authors study the diagnosis and the prognosis in 25 children with systemic lupus erythematosus and in 22 children with dermatomyositis. The most frequent signs when the diagnosis is established are articular involvements, erythema of the face, renal disorders and the presence of LE cells. The symptoms are often misleading, which explains the long delay between the first sign of the disease and the time of diagnosis, which can vary between one month to nine years. The prognosis of this disease has improved because of modern treatments : only four patients died, three of them because of renal failure and one because of cerebral embolism, after six months, one year, three years and ten months, In dermatomyositis the most frequent signs at the time of diagnosis are skin involvements and muscular and articular symptoms. The period of time between the onset of the disease and the diagnosis is shorter : one month to four years, 19 times equal to or less than one year, because the clinical picture is quite clear. Ten times the clinical evolution lead to a complete cure, 9 times a clear improvement with sequelae was observed : retractile myelosclerosis, calcinosis and medullary compression. One patient died of intestinal ulceration and two of pneumonia aggravated by involvement of the respiratory muscles.