Two cases of cutaneous self-healing histiocytosis X in a 6 and a 16 months-old children, are reported. Clinically, the lesions were characterized by few, small, translucent and confluent papules, sometimes purpuric. The scalp lesions were seborrheic eczema-like; the elements on the groins were erosive. Systemic examinations, laboratory data and general development were completely normal. The histological pattern, in both cases, were identical of that in histiocytosis X (atypic mononuclear cells with glassy eosinophilic cytoplasm and excentric kidney-shaped nucleus). Electron microscopy of the second case showed a proliferation of histiocytes containing Langerhans bodies. Ten per cent of these cells showed multivesicular, myelin and dense laminar bodies. The latest had been specially described in other selfhealing histiocytic syndromes: congenital reticulohistiocytosis (Hashimoto-Pritzker) and generalized eruptive histiocytoma. Since the clinical behaviour and prognosis of the histiocytic proliferations can not be assumed on clinical and histological data, an attempt, of classification and understanding of these processes is presented. The possibility that histiocytosis represent a proliferation of the mononuclear-phagocytic system at different levels of cellular maturation, is considered. So it would be possible to consider: 1) Acute malignant processes: xantholeukaemia, malignant histiocytosis syndromes (histiocytic medullary reticulosis . . .), and Letterer-Siwe disease; 2) Chronic and severe processus: xanthoma disseminatum, necrobiotic xanthogranuloma, disseminated plane xanthoma, multicentric reticulohistiocytosis, localized reticulohistiocytoma of Crosti and Hand-Schüller-Christian disease; and 3) Benign processus, some of them being involutive: juvenile xanthogranuloma, regressing atypical histiocytosis, eosinophilic granuloma of the bone, Illig-Fanconi disease, Hashimoto-Pritzker disease, benign cephalic histiocytosis and generalized eruptive histiocytoma. These groups include classic X-histiocytosis, self-healing X-histiocytosis, non-X malignant histiocytosis and non-X benign or auto involutive-histiocytosis (some with intracellular lipid storage: xanthohistiocytosis) (see table II). The ultrastructural cytoplasmic markers of histiocytosis (table I) are consistent features in the accuracy of diagnosis of these conditions.