Untreated or undertreated diabetes can cause debilitating complications such as blindness and amputations. Information about the factors associated with diagnosed but untreated diabetes may help target efforts to promote appropriate treatment. Using the Medical Expenditure Panel Survey, we examine: (1) use of insulin or oral medications, (2) use of diet only, and (3) no treatment. We analyze covariates of this trichotomous outcome using multinomial logit regression. Among adults diagnosed with diabetes, 87.0% used oral medications or insulin, 10.6% used diet only, and 2.4% were untreated. Lacking a usual source of care, poor mental health, being single, and being an Asian/Pacific Islander are associated with lack of treatment. Better health, lacking a usual source of care, and attitudes against medical care are associated with using diet only. Adults with diagnosed but untreated or undertreated diabetes may be difficult for service providers to reach, and multiple strategies are needed to initiate treatment.