Malignant pleural effusion (MPE) is common with mesothelioma. We report two cases of extraordinarily viscous MPEs associated with mesothelioma. The viscosity prohibited spontaneous gravity-dependent drainage via indwelling pleural catheters. Our ex vivo experiments found very high hyaluronic acid (HA) content within the fluid. Treatment of the fluid with hyaluronidase, but not with deoxyribonucleases, significantly reduced fluid viscosity. The results provide proof that HA can contribute to high viscosity of pleural fluid in mesothelioma. Research into strategies of counteracting HA properties in the management of MPEs may provide further insight.