The poor often experience illness and the treatment costs are high and even catastrophic for the poor. This paper reports the extent of illness, treatment‐seeking behaviour and out of pocket healthcare expenditures and the determinants of treatment‐seeking behaviour and healthcare expenditures among the urban poor living in Delhi. A total of 2,998 households participated in the study. Socio‐demographic details, illness experiences (episodic illness in the past 3 months, hospitalisation in the past 1 year and any chronic illness), treatment seeking and healthcare expenditures were collected for all household members through a pretested, interviewer‐administered questionnaire. Logistic regressions were carried out for factors associated with treatment‐seeking choices. Multiple linear regressions were carried for factors associated with out of pocket expenditures (OOPE). Of the total 15,218 household members (of the 2,998 households), 4,052 (26.6%) experienced episodic illness (mainly fever, respiratory illnesses, food‐ and waterborne diseases and eye infections) in the past 3 months, 230 (1.5%) were hospitalised and 976 (6.4%) have chronic illness (mainly hypertension, diabetes, arthritis and respiratory problems). Of the 2,998 households, 2,225 (74.2%) households reported at least one event of illness. Unqualified practitioners were the main source of care for episodic illnesses. Perceived seriousness of the illness, having Employees State Insurance Scheme (ESIS) benefit, higher educational status of the head of the household, higher monthly household incomes, belonging other backward castes and settled‐migrant status led to seeking formal care. Dengue was the main reason for hospitalisation. Government including ESIS hospitals were mainly utilised for hospitalisation. Healthcare expenditures were higher for private healthcare. Possession of mandatory health insurance was protective against OOPEs. OOPEs were more for the men/boys and for the young. Improving access to government healthcare services is important. Extending the ESIS to the unorganised workers including urban poor migrants should be considered so as to bring them under mandatory social protection.