Progress towards universal primary education by 2015 is monitored by primary school enrolment rates, survival rates to grade 5 and youth literacy rates. For the period 2000-2004, official statistics show Nairobi’s primary school gross and net enrolment rates as being much lower than any other province in Kenya. Nairobi’s informal settlements (slums) are estimated to be home to over 50% of the city’s population and are largely served by low cost p r i v a t e , c ommu n i t y a n d n o n - f o rma l s c h o o l s ma n a g e d b y i n d i v i d u a l entrepreneurs, communities and non-governmental organizations. This paper examines Nairobi’s slum and non-slum primary school enrolment rates and their under-reporting in government statistics. The study was conducted in two slum settlements (Korogocho and Viwandani) as well as in two non-slum sites (Jericho and Harambee) in Nairobi. A total of 13,882 children aged 5-19 years were identified in 2005 regardless of whether or not they were in school in the four geographically-defined study sites. The study used retrospective schooling history data for six years (2000-2005) collected within the longitudinal framework of the Nairobi Urban Health a n d D e m o g r a p h i c S u r v e i l l a n c e S y s t e m (w w w . a p h r c . o r g / R e s e a r c h / DSS.html). Data were collected on a range of issues in modular form using five questionnaires, each addressing a specific theme. For children aged 5, 6, 7, 8, and 9 years in 2005, only data for one (2005), two (2005 and 2004), three (2005-2003), four (2005-2002) and five (2005-2001) years were collected starting in 2005, respectively. For those aged 10 to 19 years, data for six years (2000-2005) were collected. We use these data to calculate the gross and net enrolment rates as well as the Gender Parity Index (GPI) and rate of underreporting of these enrolment rates in government statistics for the four sites in 2005. The results show high gross enrolment ratios of 100.7% and 106.8% for the slum and non-slum sites, respectively, for 2005. The net enrolment ratio is equally high at 83.3% and 94.5% for the slum and non-slum areas for the same year. These are however under-reported in government statistics by up to 23.3% for all the four sites. At an average of 1.08 for the period 2000-2005, the GPI values in the slums are in favor of girls while the opposite is true for the non-slum sites at an average GPI value of 0.93 in favor of boys.