Treatment dropout is an important concern in eating disorder treatments as it has negative implications for patients’ outcome, clinicians’ motivation, and research studies. Our main objective was to conduct an exploratory study on treatment dropout in a two-part web-based cognitive behavioral therapy with asynchronous therapeutic support. The analysis included 205 female patients with eating disorders. Reasons for dropout, treatment experiences, and predictors of dropout were analyzed. Overall treatment dropout was 37.6%, with 18.5% early dropout (before or during treatment part 1) and 19.0% late dropout (after part 1 or during part 2). Almost half of the participants identified personal circumstances as reason for dropout. The other participants mostly reported reasons related to the online delivery or treatment protocol. Predictors of early dropout included reporting less vigor and smoking at baseline and a longer average duration per completed treatment module of part 1. Late dropout was predicted by reporting less vigor at baseline and uncertainty about recommendation of the treatment to others after completion of treatment part 1. Generally, the web-based treatment and online therapeutic support were evaluated positively, although dropouts rated the treatment as significantly less helpful and effective than completers did.