Aim: This study aimed to investigate the importance of duration of illness and severity of underweight in anorexia nervosa patients. The hypothesis being that a shorter total duration of illness and a less severe degree of underweight are associated with (1) less eating disorder pathology at admission, and (2) predictive of greater improvement at discharge. Method: The sample (N = 109) consisted of patients admitted between 2013 and 2019. The study design was naturalistic and data were routinely collected as part of admission and discharge procedures, and treatment-as-usual was offered. Data were obtained from records and self-report questionnaires. Changes in body weight, body mass index and measures of eating pathology were examined pre- and post-admission. A multilinear regression and a logistic regression were performed to identify the predictive value of duration of illness and low weight at admission on improvement of eating disorder pathology. Results: Neither duration of illness nor the severity of low weight at admission were found to be significantly correlated with eating pathology at admission, nor did these variables significantly predict improvement in eating disorder pathology or recovered status. Conclusions: Results presented argue against the importance of utilizing duration of illness as a main criterion in staging or classifying patients as severe and enduring. Findings suggested that even individuals with a long duration of illness, as well as individuals at a very low weight at admission, may achieve a favorable outcome.