Nanostructured zinc oxide (ZnO) has received much attention due to its biological and medical applications, where detailed knowledge about particle sizing, aggregation propensity and its related hazards are crucial. Herein, the aggregation propensity and dissolution behavior of ZnO nanoparticles in aqueous medium (PBS) were studied as a function of concentration and further correlated with its toxicity in RAW 264.7 murine macrophages. Fast formation of smaller aggregates having high dissolution rate was observed at low concentration ZnO (LC-ZnO). Compared to high concentration ZnO (HC-ZnO) aggregates, the LC-ZnO aggregates were highly pronounced in terms of reactive oxygen species generation and exerting cell apoptosis, ascribed to the secondary size effect, size-dependent cellular uptake and ion solubility. This study outlines the nanoparticle concentration as a key factor in scaling its aggregation, dissolution tendency and also emphasizes the accounting of ingested nanomaterials long-term fate inside the cells. (C) 2014 Elsevier B.V. All rights reserved.