To the Editor: In their Review, Dr van der Lee and colleagues1 found wide variability in the reported prevalence of chronic health conditions in childhood, largely a result of different definitions. Only 1 of the 64 articles included in the Review reported the percentage of children with more than 1 condition.2 Multimorbidity, the co-occurrence of multiple chronic conditions in 1 individual, is increasingly recognized as having a major effect on patient outcomes and health care costs, posing a significant challenge to current models of health care.3