BACKGROUND: Because atopic dermatitis (AD) is a relapsing remitting disease, assessing long-term control is important. Well-controlled weeks (WCWs) have been used to assess asthma long-term control but have never been validated for AD. OBJECTIVES: We sought to assess the feasibility, validity, and interpretability of WCWs in patients with AD. METHODS: Three studies of patients with moderate-to-severe AD, including 4 to 6 months of daily/weekly symptom and treatment use data, were evaluated (study A, n = 336; study B, n = 60; and study C, n = 224). WCWs were defined by worsening symptoms and increased medication use. Feasibility, construct validity, and interpretability of WCWs were determined by assessing missing data, association with validated AD outcomes, and floor and ceiling effects. Analysis used linear and logistic regression. RESULTS: WCWs were feasible to collect: 95.2% (study A) and 94.7% (study B) contributed data for at least half of the weekly data points, and 93.2% and 88.7% contributed to all data points up to 4 months. WCWs were significantly associated with validated AD severity instruments, including patient-orientated outcome measures and objective signs (Eczema Area and Severity Index, Three Item Severity Score, and Six Signs, Six Areas Atopic Dermatitis Scale). The odds of experiencing a WCW if AD severity was clear/mild was 5.8 (95% CI, 3.5-9.7), 1.9 (95% CI, 0.8-4.4), and 8.1 (95% CI, 4.5-14.6) in studies A, B, and C, respectively. WCWs were associated with ceiling effects: 31.6% (study A) and 37.5% (study B) of participants had no WCWs more than 90% of the time. CONCLUSIONS: WCWs are valid and feasible for measuring long-term control in AD trials. However, ceiling effects and burden of data collection can limit use.