Prevalence of dementia is expected to increase three- to four-fold in the next 50 years. In 1986, New York State established the Alzheimer's Disease and Other Dementias Registry, one of a few such registries in the United States. We identify surveillance challenges within the Registry. Data quality — specifically, the attributes of completeness and accuracy — is the primary challenge to the New York State dementias registry. Completeness may be undermined when hospitals and nursing homes fail to report data, and hospital charts do not record dementia diagnoses. Failure to record diagnoses may occur because of diagnosis uncertainty, perceived stigma, clinical attention on the primary reason for hospitalization, and financial disincentives. Dementia is well recorded in nursing home data because care planning requires frequent resident evaluations. The accuracy of recording specific forms of dementia is limited because coding terminology has not kept pace with physicians' perspectives on dementias. Hospitals and nursing homes document dementia and comorbidities more effectively among frail individuals and those with advanced dementias than among individuals who appear to be relatively healthy. One way to overcome challenges of data quality is to form partnerships with organizations that have expertise in managing medical records and coding dementias. As medical advances make early diagnoses more possible outside the hospital or nursing home setting, we will need to redesign the current surveillance system to capture this additional dementia data and ensure a representative system.