Objective: To investigate natural- and unnatural-cause mortality at different follow-up time points in Taiwanese patients with anorexia nervosa (AN) and bulimia nervosa (BN). Method: In this longitudinal cohort study, 330,393 patients, including 2143 patients with AN, 13,590 with BN, and 20 times as many respective non-AN and non-BN patients, were followed up for 16 years. We performed conditional Cox regression survival analysis to examine the risk of mortality in the AN and BN groups relative to the comparison group. Results: A total of 1242 patients died, including 101 and 343 patients with AN and BN, respectively. Mortality rates for AN and BN were 5.42 and 2.90 deaths per 1000 person-years, respectively. Compared with the non-AN group, the AN group had a significantly higher risk of both natural- and unnatural-cause mortality, and the BN group had a significantly higher risk of unnatural-cause mortality. Suicide was the most common cause of death, and suicide risk was significantly higher in both the AN and BN groups. All-cause mortality risk was the highest at the beginning of follow-up and markedly declined in the AN group. In the BN group, all-cause mortality risk was lower but stable at follow-up. The risk of unnatural-cause mortality remained high throughout the follow-up period for both the groups. Conclusions: Early detection and treatment for associated physical problems in patients with AN are crucial. Regular monitoring for unnatural-cause mortality events (mainly suicide) in AN and BN over time is also crucial. Public Significance: AN had a significantly higher risk of both natural- and unnatural-cause mortality and BN had a significantly higher risk of death from unnatural causes. All-cause mortality risk was highest at the beginning of follow-up in AN, but unnatural-cause mortality risk remained high throughout the follow-up period for both groups. Our findings imply that early detection and treatment in AN and regular monitoring for unnatural-cause mortality events in AN and BN are crucial.