Background The general health of children of parents with mental illness is overlooked. Aims To quantify the difference in healthcare use of children exposed and unexposed to maternal mental illness (MMI). Method This was a retrospective cohort study of children aged 0-17 years, from 1 April 2007 to 31 July 2017, using a primary care register (Clinical Practice Research Datalink) linked to Hospital Episodes Statistics. MMI included non-affective/affective psychosis and mood, anxiety, addiction, eating and personality disorders. Healthcare use included prescriptions, primary care and secondary care contacts; inflation adjusted costs were applied. The rate and cost was calculated and compared for children exposed and unexposed to MMI using negative binomial regression models. The total annual cost to NHS England of children with MMI was estimated. Results The study included 489 255 children: 238 106 (48.7%) girls, 112 741 children (23.0 %) exposed to MMI. Compared to unex-posed children, exposed children had a higher rate of healthcare use (rate ratio 1.27, 95% CI 1.26-1.28), averaging 2.21 extra contacts per exposed child per year (95% CI 2.14-2.29). Increased healthcare use among exposed children occurred in inpatients (rate ratio 1.37, 95% CI 1.32-1.42), emergency care visits (rate ratio 1.34, 95% CI 1.33-1.36), outpatients (rate ratio 1.30, 95% CI 1.28-1.32), prescriptions (rate ratio 1.28, 95% CI 1.26-1.30) and primary care consultations (rate ratio 1.24, 95% CI 1.23-1.25). This costs NHS England an additional £656 million (95% CI £619-£692 million), annually. Conclusions Children of mentally ill mothers are a health vulnerable group for whom targeted intervention may create benefit for individuals, families, as well as limited NHS resources.