Small amyloid-beta (A beta) oligomers are believed to be key pathogenic species in Alzheimer's disease (AD). One suggested toxicity mechanism is the detergent model where oligomers remove lipid molecules from the bilayer. Senile plaques of AD patients also accumulate a 1:1 ratio of cholesterol/A beta. What are the dominant structures of small A beta 42 oligomers with cholesterol molecules in aqueous solution? Here, we answer this question by performing atomistic replica exchange molecular dynamics simulations of A beta 42 dimers and trimers. Our simulations demonstrate that the interactions with cholesterol molecules change completely the energy landscape of small A beta 42 oligomers. This result shows that simulations in the bulk solution cannot recapitulate aggregation in the brain extracellular space.