Background: Lockdown implemented to prevent the COVID-19 spread resulted in marked changes in the lifestyle. The objective of the current study was to assess the impact of lockdown measures on a cohort of eating disorder (ED) patients being followed as part of an ongoing naturalistic treatment study. Methods: Ninety-nine patients aged 18 or older, currently or previously, in treatment at a Portuguese specialized hospital unit were contacted by phone and invited to participate in the current survey. Fifty-nine agreed to be interviewed by phone, and 43 agreed to respond to a set of self-report measures of ED symptoms, emotion regulation difficulties, clinical impairment, negative urgency, and COVID-19 impact, during the week after the end of the lockdown period. Results: Data showed that of the 26 patients currently in treatment: 8 remained unchanged (31%), 7 deteriorated (27%), and 11 reliably improved (42%). Of the 17 participants not currently in treatment: 3 deteriorated (18%), 9 remained unchanged (53%), and 5 (29%) improved after the lockdown period. The Coronavirus Impact Scale showed that most patients considered their routines moderately or extremely impacted, experienced stress related to coronavirus, and showed difficulty in maintaining physical exercise and feeding routines. Results suggest that higher impact of COVID-19 lockdown was significantly correlated with eating disorder symptoms and associated psychopathology, impulsivity, difficulties in emotion regulation and clinical impairment measured at post-lockdown. In addition, the impact of COVID-19 and lockdown measures on clinical impairment was mediated by difficulties in emotion regulation. Conclusions: Findings suggest that some ED patients may experience worsening of their condition, especially if associated with difficulties in emotion regulation, and these difficulties might be exacerbated in the context of a stressful crisis and lockdown measures, highlighting the need for intervention strategies to mitigate its negative impact.