Introduction: NET (Neuroendocrine Tumours) are a heterogeneous group of tumours. It is important to classify these heterogeneous NET, both for patient care and to be able to compare studies. A NET is classified by grading and immunohistochemical staining with markers. Tumour markers like Chromogranine A (CgA) can also circulate in the bloodstream and can be used as a diagnostic marker and for follow-up. The relationship with the histological presence is unknown. Since the WHO classification from 2010, the grade is defined by the mitotic count and the Ki-67 index. The Ki-67 index is usually estimated by the pathologist. However there are also digital methods to measure the Ki-67 index. This study examined the correlation between the serum Chromogranin A and the immunohistochemical expression of CgA, the correlation between the mitotic count and the Ki-67 and if computer-assisted image analysis can be of value in the grading of NET. Design: 43 biopsies of patients diagnosed with NET, who visited the Antoni van Leeuwenhoek hospital between 1996 and 2008, were analysed. The CgA expression, the mitotic count and the Ki-67 immunostained cells were studied. The serum values of CgA were collected from patient files. On 7 samples there was no expression of Ki-67 staining, so no pictures of these were taken. 34 Ki-67 immunostained slides were circulated among 2 pathologists for visual evaluation. Computer-assisted image analysis was performed using ImageJ. Ki-67 index was categorized in three groups ≤ 2%, ; 3-20%, ; >20%. The Ki-67 index determined by both pathologists and digital analysis were compared in cross tables. The correlation was calculated with the Spearman correlation test. Results: The tumour cells immunostained in the majority of the cases (40/43) 100% with CgA. Almost in half of the cases (15/34) the grade based on the mitotic count per 2mm2 is different from the grade based on the Ki-67 index. The Ki-67 was ≤2% in 81% of the slides, 3-20% in 12% of the slides and >20% in 7% of the slides. The serum values varies from 27 to 24589 µg/l. The results of Pathologist 1 correlates better with ImageJ than the results of pathologist 2. (correlation coëfficiënt: 0.838 en 0.379). The pathologists correlate moderately with each other. (correlation coëfficiënt: 0.484). Conclusion: Serum values of the tumour marker Chromogranin A (CgA) does not correlate with the degree of CgA in tumour cells in a random selected group of patients with NET. Almost in half of the cases the grade based on the mitotic count per 2mm2 is different from the grade based on the Ki-67 index. The Ki-67 indexes assessed by the pathologists, do not correlate well with each other. The Ki-67 index determined with ImageJ correlates well with the Ki-67 index of one of the pathologists. These results show that further standardization of the Ki-67 index is necessary.