A growing number of diseases are being linked to protein misfolding and amyloid formation. Recently, p53 was also shown to associate into amyloid aggregates, raising the question of whether cancer development is associated with protein aggregation as well. However, a lack of suitable tools has hampered the evaluation of their clinical relevance. Herein, we report an enzyme-linked-immunosorbent-assay (ELISA) system based on a polyionic, high-molecular-weight ligand that specifically captures aggregated oligomers and amyloid proteins. We proved that naturally occurring tetramers of p53 are not bound, but high-molecular-weight aggregates are bound and subsequently detected. For the first time, this assay allows the quantitative detection of p53 aggregates from cell lysates, which was demonstrated using 22 ovarian-cancer cell lines as well as 7 patient-derived tumor tissues. The levels of p53 aggregates within the missense-mutated tissue samples varied more than 12-fold. This simple, robust method allows studying the abundance and clinical relevance of protein aggregates. This could help our understanding of the role of protein misfolding in cancer or even in predicting therapy responses to aggregation-targeting drugs.