The structure and ultrastructure of the villi of small intestinal mucosa was examined in 237 duodenal or jejunal biopsies taken from children with active celiac disease and during gluten-free diet. All biopsies were processed for light and scanning electron microscopy. Conventional histology showed four different morphological aspects: total and subtotal villous atrophy in patients on unrestricted diet, partial villous atrophy and normal mucosa during gluten-free diet. Scanning electron microscopy demonstrated that in active celiac disease the severity of the intestinal lesions was related to individual vulnerability to gluten. Our results showed that during dietary treatment the process of mucosal healing was constant and strictly time-dependent. Furthermore, the ultrastructural examination has been relevant in evaluating the evolution of the villous regeneration. In this study a classification regarding the healing process of the small intestinal mucosa correlated with the time of start of dietary therapy is proposed.