BACKGROUND: In a population-based birth cohort, we aimed to identify longitudinal trajectories of atopic dermatitis (AD) during childhood using data from different sources (validated questionnaires and healthcare records), investigate impact of different AD definitions on such trajectories and their relationships with various risk factors. METHODS: Of the 1184 children born into the study, 1083 had information on current AD on at least three follow-ups from birth to age 11 years and were included in the analysis for parentally-reported AD (PRAD). Data were transcribed from healthcare records of 916/1184 children, for the analysis of doctor-diagnosed AD (DDAD). We also derived composite definition (CDAD; at least 2 of 3 features: PRAD, DDAD, current use of AD treatment). Using latent class analysis (LCA), we determined longitudinal profiles of AD using the three definitions (PRAD, DDAD CDAD). FLG genotype was available for 803 Caucasian participants. RESULTS: For PRAD, LCA identified four AD classes ("No AD", "Persistent", "Early-onset remitting"" and "Late-onset"). For DDAD and CDAD, the optimal number of phenotypes was three ("No AD", "Persistent" and "Early-onset remitting"). Although AD classes at population level appeared similar in different models, a considerable proportion of children (n=485, 45%) moved between classes. The association with FLG genotype, atopic diseases, and early-life risk factors were inconsistent across different definitions, but the association with oral food challenge-confirmed peanut allergy was similar, with a 9 to 11-fold increase amongst children in the Persistent AD class. In a CDAD model, compared to Early-onset remitting class, those with Persistent AD were significantly more likely to have (at age 3 years) moderate/severe AD (OR=11.6, [95% CI 1.7-80.2]), polysensitisation (5.2, [1.3-21.2]), and current wheeze (4.8, [1.4-16.6]), and were less likely to be breastfed (0.2, [0.05-0.8]). CONCLUSIONS: Standardised composite definitions of AD may help define AD cases with more precision and identify more consistent long-term trajectories.