The transsphenoidal approach is a less invasive and safer procedure for removing pituitary adenomas. However, this procedure becomes extremely difficult when the tumour consistency is fibrous as encountered in about 10% of pituitary adenomas. In this study, we investigated predicting factors of tumour consistency in magnetic resonance images (MRIs). MRIs of two groups, twenty-one soft and five firm (fibrous) adenomas, were retrospectively evaluated and compared in respect of tumour consistency. To compare the two groups objectively, tumour densities on MRI films and percentage of collagen content on operative specimens were expressed as numerical data using NIH-image. The relationships between collagen content and T1-weighted images, T2-weighted images, grade of enhancement effect, and heterogeneity of enhancement were investigated. Signal intensities on T1-weighted images were not correlated with tumour consistency, whereas those on T2-weighted images were significantly correlated with the percentage of collagen content. Adenomas, showing lower signal intensities on T2-weighted images, contained more collagen. On enhanced images, homogeneously enhanced adenomas tended to include more collagen, even though the grade of enhancement effect showed only weak correlation with the tumour hardness. MRIs give us useful information on tumour consistency. Adenomas may be firm and fibrous if they show low signal intensities on T2-weighted images and homogeneous enhancement. To remove such tumours, a long sized and small-calibred ultrasonic aspirators applicable to transsphenoidal approach must be prepared and multistaged operations may be more than likely needed.