Objectives: To elucidate the importance of clinically diagnosed cerebral comorbidity in idiopathic normal-pressure hydrocephalus (INPH) and its effect on improvement after shunt surgery as well as concordance with parenchymal pathological changes described in frontal cerebral biopsy specimens. Methods: In 28 consecutive patients diagnosed with INPH and shunted according to clinical, radiological and cerebrospinal fluid dynamic criteria, concomitant disorders were carefully registered, with special emphasis on cerebrovascular disease (CVD) and possible Alzheimer's disease. During shunt surgery, a frontal cerebral biopsy specimen was obtained and subsequently analysed for pathological changes. Results: One or several concurrent disorders were present in 89% of the patients, most often CVD (n = 17) and possible Alzheimer's disease (n = 12), of which eight patients presented both, diagnosed according to the criteria of the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association. The shunt success rate was 33%. A clear tendency towards increasing prevalence of CVD or Alzheimer's disease was found in the subgroups with no improvement or clinical deterioration compared with the patients improving after shunt surgery. The presence of CVD tended towards an unfavourable shunt outcome. The pathological parenchymal changes reflected the clinical diagnoses of comorbidity, and were described in about half of the biopsy specimens, with Alzheimer's disease (n = 7) and vascular changes (n = 7) being the most common findings. However, no significant correlation was found with the clinical diagnoses of Alzheimer's disease and CVD. The presence of cerebral comorbidity, whether diagnosed clinically or by brain biopsy, did not preclude clinical improvement after shunt operation. Conclusions: A high prevalence of CVD and Alzheimer's disease was found in patients shunted for INPH, which was reflected, although less commonly, by similar neuropathological biopsy findings. No significant correlation was found between the presence of comorbidity and shunt outcome. The findings support the perception of INPH as a multiaetiological clinical entity, possibly overlapping pathophysiologically with CVD and Alzheimer's disease.