The periodontal health of smokers and non-smokers with non-insulin-dependent diabetes mellitus (NIDDM) and non-smokers with periodontitis who were not suffering from a systemic disease was assessed. The investigation was carried out on 60 adult subjects. Levels of blood glucose, glycosylated haemoglobin and immunoglobulins G, A and M were determined, together with the plaque index, gingival index, probing pocket depths and clinical attachment level. Periodontitis was more severe in smokers and non-smokers with NIDDM than non-smokers without NIDDM, and the periodontal condition (clinical attachment level, probing pocket depth and gingival bleeding) was better in non-smokers with NIDDM than smokers with NIDDM. The results suggest that diabetes and smoking are high-risk factors for periodontal disease.