This study investigated the effects of diabetes mellitus (types I and II) on human salivary gland function compared to healthy age-matched controls. The results have shown that both type I and type II diabetic patients secrete significantly (p < 0.05) less resting and stimulated saliva compared to healthy age-matched controls (AMC). It was also found that the diabetic patients have an increased resting and stimulated salivary protein concentration compared to healthy participants. However, the secretory capacity (stimulated minus resting values) was markedly reduced compared to controls. The level of calcium (Ca2+) in the saliva of diabetic patients was significantly (p < 0.05) elevated compared to the AMC. In contrast, the levels of magnesium (Mg2+), zinc (Zn2+) and potassium (K+) in the saliva of diabetic patients were significantly (p < 0.05) reduced compared to the values obtained in AMC. These results indicate that diabetes mellitus can lead to marked dysfunction of the secretory capacity of the salivary glands. In these patients a modified fluid, organic and inorganic salivary secretion may be responsible for the increased susceptibility to oral infections and impaired wound healing described by others in the literature.