To investigate whether cesarean delivery increases a child’s risk of atopic dermatitis (AD) by the age of 4 by using data from a large integrated health care delivery system. It has been hypothesized that cesarean delivery may disrupt maternal–infant microbial transfer, thereby affecting immune system development and increasing the risk of atopic dermatitis in children.Mothers who had a singleton live birth at a Kaiser Permanente North California or contracting hospital between January 1, 2005, to January 1, 2014 and whose child had continuous enrollment in the Kaiser Permanente North California system for at least 4 years were included in the study (n = 173 105). Among this birth cohort, a subsample of births (n = 102 327) occurring mostly after 2008 because of the adoption of electronic health records included more detailed information of cesarean delivery conditions (start of labor, timing of membrane rupture, and cesarean delivery indication).In this observational study, the researchers collected maternal and child information using these data sources: electronic health record, pharmacy databases, state birth records, and a prospectively collected survey regarding breastfeeding behavior at 2 months of age. The authors used modified Poisson regression models with robust variance estimation to assess the association between cesarean delivery and atopic dermatitis overall and when stratified by demographic and labor and delivery characteristics.There were 173 105 children born to 135 102 mothers during the study time period. A total of 12% of the children (n = 20 819) met the case definition for AD, and 26.6% of the births were born via cesarean delivery. The prevalence of AD was higher among children who were male, born term, were first born and did not have an NICU admission. Mothers of children with AD were more likely to be people of color (African-American, Asian, or Pacific Islander) and have underweight prepregnancy BMI. The prevalence of cesarean delivery was higher among mothers who were older, had higher levels of education, had a higher prepregnancy BMI, and were African American race. Infants born by cesarean delivery were more likely to be born at an earlier gestational age, had a high or low birth weight, were in the NICU, and were first born. In the primary multivariate model controlling for all covariates, there was little evidence that children born via cesarean delivery were more likely to develop AD by age 4, compared with children born vaginally (risk ratio [RR]: 1.02; confidence interval [CI]: 0.99 to 1.05). Among all stratified models (AD diagnosis before or after 6 months, maternal atopy, gestational age, birth order, prepregnancy BMI, and child’s sex), the RR for cesarean delivery and AD was minimal. Cesarean delivery conditions indicative of the least exposure to maternal microbiome (ie, no labor and short interval between membrane rupture and delivery) revealed no evidence of association with AD. In addition, intrapartum antibiotics, breastfeeding, or familial factors did not strongly influence an association. Only term infants born via cesarean delivery because of a breech or other malpresentation with labor (RR: 1.07; CI 0.94 to 1.21) or because of failure to progress (RR: 1.09; CI: 1.01 to 1.17) showed modest evidence of higher risk of AD by the age of 4 years, compared with term infants born vaginally.Delivery by cesarean delivery was not associated with an increased risk of atopic dermatitis by the age of 4 years.The authors point out that this is the largest study-to-date using observational data for a US-based birth cohort regarding mode of delivery and atopic dermatitis development. The results of this study are consistent with previous research findings. The strengths of this study are the large amount of data analyzed and the prospectively collected information on cesarean delivery conditions, intrapartum antibiotic administration, breastfeeding information, and within-family characteristics (via a matched sibling analysis).