Objectives-This report expands the measures used to evaluate cause-of-death data quality by presenting a novel list of unsuitable underlying causes of death (UCOD). This list is intended to facilitate the measurement of the quality of cause-of-death reporting by medical certifiers in terms of completeness, as assessed by a UCOD that is sufficiently specific. Methods-A list of codes from the International Statistical Classification of Diseases and Related Health Problems, 10th Revision was developed to classify unsuitable UCODs defined according to three main subtypes: unknown and ill-defined causes, immediate and intermediate causes, and nonspecific UCODs. Unsuitable UCODs and the three subtypes were examined using 2018 death certificate data for both U.S. residents and nonresidents in the 50 states and the District of Columbia. Differences in the frequency of unsuitable UCODs and the subtypes were tested by age group, place of death, and state of occurrence. Trends in unsuitable UCODs and the three subtypes were also investigated by analyzing death certificate data from 2010 to 2018. Results-In 2018, 34.7% of all death records had an unsuitable UCOD: 2.2% had an unknown or ill-defined cause as the UCOD, 12.7% had an immediate or intermediate cause as the UCOD, and 19.8% had a nonspecific UCOD. Unsuitable UCODs and the subtypes varied by age group, place of death, state, and year. No trend in unsuitable UCODs from 2010 to 2013 was seen, but from 2013 to 2018, a decrease of 0.6% per year was observed, which is likely due to a similar decrease in nonspecific UCODs during the same time period. Conclusion-This novel list of unsuitable UCOD codes can be used to assess the quality of cause-of-death data over time and by other various characteristics, with further applications for efforts to improve mortality data quality.