Childhood obesity is becoming an alarming global health problem. However, the prevalence of overweight and obesity in most developing countries remain unknown due to limited data. This study aimed at assessing the prevalence of childhood obesity and underweight among Kenyan children using the internationally recommended BMI definition and to establish any association between childhood obesity and risk factors. The study addressed the following research questions; what Kenyan data are available to estimate prevalence of childhood obesity and underweight?; how has prevalence changed over time?; what do Kenyan data tell us in terms of prevalence and risk factors for obesity and underweight?; how does prevalence differ between different definitions of obesity. We used both the International Obesity Task Force (IOTF) BMI and the UK BMI definitions. Data were derived from the 1993 and 1998 Kenya Demographic and Health Surveys. The Kenya 1993 survey included 2979 subjects aged 24 to 59 months and the 1998 survey had 925 subjects aged 24 to 35 months. Differences between observed and expected prevalence for underweight, overweight and obesity relative to the UK 1990 BMI reference data were tested for significance by Chi-Squared Goodness of–fit–Test. The prevalence of underweight was significantly higher than expected relative to UK 1990 BMI definition for both 1993 and 1998 surveys. There was no significant difference in the prevalence of overweight among Kenyan children for 1993 and 1998 relative to both UK 1990 BMI and IOTF BMI definitions. There was no significant difference in obesity prevalence relative to UK 1990 and IOTF BMI definitions between 1993 and 1998 surveys (Chi-Squared Tests, P > 0.1). Both obesity and underweight co-exist among Kenyan children.