After successive passes through a syringe needle, poly(ethylene oxide) (PEO) in dilute aqueous solution undergoes chain aggregation. The degree of association, the size, and the concentration of the species are characterized by combining static and dynamic light scattering measurements. For the highest molecular weight, polymer chains are broken in a first stage then aggregate in a second stage. Neutron scattering results show that these chains have the same conformation as in the dispersed molecular state. When the solutions are allowed to rest, the aggregates dissociate very slowly with time. After addition of sodium chloride, the dissociation is much faster and complete. We propose the aggregation to result from the concomitant increase of the instantaneous local concentration at the entrance of the syringe needle and the enhanced hydrophobic interactions to which are submitted the stretched macromolecular chains in the flow.