Polymeric nanocapsules with nanometre-thin walls offer a promising platform for controlled cellular delivery of therapeutic or diagnostic agents. Therefore, their biocompatibility is crucial for future applications in the human body. However, there is little knowledge about their interaction with biological systems. In this study, polymeric nanocapsules containing different amounts of lipids and representing different scenarios for handling and storing nanocapsules are investigated. We find that all nanocapsules in our study can enter human cells and the presence of an outer lipid shell facilitates the process. These nanocapsules do not inhibit cell proliferation at concentrations up to 200 μg mL(-1) of nanocapsules. No cellular ROS, apoptosis or cell cycle perturbation is detected at this dose. These comprehensive examinations demonstrate that polymeric nanocapsules are promising nano-carriers for cellular delivery.