The practice of appropriate health seeking has a great potential to reduce the occurrence of severe and life-threatening child illnesses. We assessed the influence of socio-demographic, economic and disease-related factors in health care seeking for child illnesses among slum dwellers of Nairobi, Kenya. A survey round of the Nairobi Urban Demographic Surveillance System (NUDSS) generated information on 2-week child morbidity, illness symptoms, perceived illness severity and use of modern health services. During this round of data collection, interviewers visited a total of 15,174 households, where 3015 children younger than 5 years lived. Of the 999 (33.1%) children who were reported to have been sick, medical care of some sort was sought for 604 (60.5%). Lack of finances (49.6%) and a perception that the illness was not serious (28.1%) were the main reasons given for failure to seek health care outside the home. Health care seeking was most common for sick children in the youngest age group (0-11 months). Caretakers sought medical care more frequently for diarrhoea symptoms than for coughing and even more so when the diarrhoea was associated with fever. Perception of illness severity was strongly associated with health care seeking. Household income was significantly associated with health care seeking up to certain threshold levels, above which its effects stabilized. Improving caretaker skills to recognize danger signs in child illnesses may enhance health-seeking behaviour. Integrated Management of Child Illnesses (IMCI) programmes must be accessible free of charge to the urban poor in order to increase health care seeking and bring about improvements in child survival.