Most pituitary adenomas, also termed pituitary neuroendocrine tumors (PitNETs), are benign in nature and can be treated effectively by surgical resection, medical treatment and in special cases by radiotherapy. However, invasive growth can be an important feature of a more aggressive behavior and adverse prognosis. Extension of pituitary adenomas into the cavernous sinus can be categorized according to the Knosp criteria on magnetic resonance imaging (MRI). Comparative analyses of MRI features and intraoperative findings of invasive growth regarding different clinical factors are still scarce. We performed a retrospective single-center analysis of 764 pituitary adenomas that were surgically treated between October 2004 and April 2018. Invasive growth was assessed according to the surgical reports and preoperative MR imaging (Knosp criteria). Clinical data such as patient age at diagnosis and gender, histopathological adenoma type as well as extent of resection were collected. Invasive features on MRI were seen in 24.4% (Knosp grade 3A - 4, 186/764) of cases. Intraoperatively, invasion was present in 42.4% (324/764). Complete resection was achieved in 80.0% of adenomas and subtotal resection in 20.1%. By multivariate analysis, invasion according to intraoperative findings was associated with the sparsely granulated corticotroph (SGCA, p=0.0026) and sparsely granulated somatotroph (SGSA, p=0.0103) adenoma type as well as age (p=0.0287). Radiographic invasion according to Knosp grades 3A-4 correlated with age (p=0.0098), SGCAs (p=0.0005), SGSAs (p=0.0351) and gonadotroph adenomas (p=0.0478).Both criteria of invasion correlated with subtotal resection (p=0.0001, respectively). Both intraoperative and radiographic signs of invasive growth are high-risk lesions for incomplete extent of resection and occur more frequently in older patients. A particularly high prevalence of invasion can be found in the sparsely granulated corticotroph and somatotroph adenoma types. Cavernous sinus invasion is also more common in gonadotroph adenomas. Usage of the Knosp classification is a valuable preoperative estimation tool.