AIMS: This study aims to assess the prevalence, awareness, treatment, and control of hypertension in two major slums in Nairobi, Kenya. METHODS: We use data from a cross-sectional population-based survey, conducted in 2008-2009, involving a random sample of 5190 (2794 men and 2396 women) adults aged 18 years and older resident in both slums. RESULTS: Overall, the prevalence (weighted by sampling and response rates) of hypertension (SBP ≥140  mmHg and/or DBP ≥90  mmHg and/or antihypertensive medication) was 12.3% (12.7% in women and 12.0% in men). The overall level of awareness (having been previously informed of hypertensive status by a health professional) among hypertensives was 19.5% (30.7% in women and 10.8% in men). About 47% (44.9% in women and 50.9% in men) of those who were aware of being hypertensive reported being on antihypertensive treatment in the 1 year preceding the survey. Among those who reported being on treatment, only 21.5% (14.4% in women and 35.7% in men) had their hypertension controlled to levels below 140/90  mmHg. Hypertension control among all hypertensives was below 3%. CONCLUSION: Our findings suggest that hypertension is common in the slums, but the rates of awareness, treatment, and control are low. However, once people are aware of their hypertension, most seek treatment. This indicates that the best gains in treatment can be made when awareness is raised. Overall, there is urgent need to implement strategies that improve prevention, detection, and access to effective treatment in these neglected populations.