The purpose if this study was to investigate the positive predictive value and sensitivity of the ICD-10 code G12.2, which is used to identify patients who have possibly died from ALS. All patients with a motor neuron disease diagnosis code during the study period (2001-2005) were identified using administrative data. South Carolina death certificate data were used to assess the positive predictive value and sensitivity of the ICD-10 code G12.2. Two hundred and seventy known cases of ALS linked to the death certificate data file. G12.2 was coded as either the underlying or contributing cause of death for 229 cases, sensitivity = 85%. There were 318 deaths due to ALS identified by the G12.2 code where a medical record was available for review. Of those, 205 contained information supporting the diagnosis of ALS, positive predictive value = 65%. This evaluation raises questions concerning the validity of using mortality data in forming epidemiological conclusions in this patient population. However, it does appear that mortality data can be used in the development of case-finding algorithms to identify ALS patients through the use of administrative data sets.