This Healthcare Cost and Utilization Project (HCUP) Statistical Brief presents statistics on the most frequent principal diagnoses among nonmaternal, nonneonatal inpatient stays using the 2018 National Inpatient Sample (NIS). First, the number of stays, mean cost per stay, and aggregate costs are presented for the most frequent principal diagnoses. Second, the distribution of stays for the most common diagnoses by select patient and hospital characteristics is shown. Finally, the top principal diagnoses by sex-age group are identified. Because of the large sample size of the NIS data, small differences can be statistically significant. Thus, only differences greater than or equal to 10 percent are discussed in the text.