The oral manifestations are described of 43 patients with previously undiagnosed non-insulin dependent diabetes mellitus (NIDD) who presented to an Oral Medicine Unit in a 24-month period. The diagnosis of diabetes mellitus was confirmed by measurement of fasting blood glucose and where necessary by a glucose tolerance test. Sixteen patients had burning mouth syndrome, ten patients had unusually prolonged fungal and/or bacterial infections, nine patients had a nonspecific condition such as altered taste, six patients had an oral mucosal lesion such as erosive lichen planus, one patient had sialosis and one patient complained of sialorrhoea. The frequency of undiagnosed NIDD was twice that expected among the general population. Treatment directed at improving glycemic control also produced resolution of the patients' oral symptoms in most cases.