Aflatoxins contaminate approximately 25% of agricultural products worldwide. They can cause liver failure and liver cancer. Kenya has experienced multiple aflatoxicosis outbreaks in recent years, often resulting in fatalities. However, the full extent of aflatoxin exposure in Kenya has been unknown. Our objective was to quantify aflatoxin exposure across Kenya. We analysed aflatoxin levels in serum specimens from the 2007 Kenya AIDS Indicator Survey - a nationally representative, cross-sectional serosurvey. KAIS collected 15,853 blood specimens. Of the 3180 human immunodeficiency virus-negative specimens with ≥1 mL sera, we randomly selected 600 specimens stratified by province and sex. We analysed serum specimens for aflatoxin albumin adducts by using isotope dilution MS/MS to quantify aflatoxin B1-lysine, and normalised with serum albumin. Aflatoxin concentrations were then compared by demographic, socioeconomic and geographic characteristics. We detected serum aflatoxin B1-lysine in 78% of serum specimens (range = <LOD-211 pg/mg albumin, median = 1.78 pg/mg albumin). Aflatoxin exposure did not vary by sex, age group, marital status, religion or socioeconomic characteristics. Aflatoxin exposure varied by province (p < 0.05); it was highest in Eastern (median = 7.87 pg/mg albumin) and Coast (median = 3.70 pg/mg albumin) provinces and lowest in Nyanza (median = <LOD) and Rift Valley (median = 0.70 pg/mg albumin) provinces. Our findings suggest that aflatoxin exposure is a public health problem throughout Kenya, and it could be substantially impacting human health. Wide-scale, evidence-based interventions are urgently needed to decrease exposure and subsequent health effects.