The degree of defective neutrophil chemotaxis in patients with Down's syndrome (DS) and its relationship to the severity of periodontal disease were studied. Fourteen patients with DS and 14 healthy controls were examined. Oral hygiene, gingival inflammation, and pocket depths were measured in clinical surveys. Bone loss was evaluated on the oral radiographs. Neutrophil chemotaxis was measured by the agarose plate method and the Boyden chamber method. The chemotactic index of the agarose plate method and the mean numbers of migrated cells of the Boyden chamber method were correlated with statistical significance (rs = 0.066, P less than 0.01). DS patients showed significantly lower chemotaxis than healthy volunteers with both methods. No difference was shown between the two groups in the random migration of the neutrophils. From the oral radiographic analysis, the DS patients exhibited various prevalence of bone loss which was inversely proportional to the chemotactic index and a significant correlation between them was shown (rs = -0.612 P less than 0.05). A significant correlation was also found between the age of the patient and the prevalence of bone loss (rs = 0.591 P less than 0.05). These results indicate that defective neutrophil chemotaxis influences the progression of periodontal disease in DS patients.