THE AUTHORS EVALUATED a series of proven intracranial cystic lesions prospectively. The relative signal intensities of these lesions on both T1- and T2-weighted sequences were correlated with the composition and viscosity of the cystic contents. Specimens were collected from 51 patients by cyst aspiration or at the time of surgery. Once a specimen was obtained, it was immediately sent for quantitative analysis of proteins, cholesterol, triglyceride, calcium, and blood by-products. In 30 patients, the cystic lesion was hypointense on T1 and hyperintense on T2 relative to white matter. The cystic content in this group of patients was a watery fluid that could be easily aspirated. In another 14 patients, the cystic lesion was either isointense or hyperintense on T1 and hyperintense on T2-weighted sequences. In this group of patients, the cystic contents were mild to moderately viscous and a wide bore needle or cannula was required for aspiration. In the remaining seven patients, the cystic contents were hyperintense (n = 4), isointense (n = 2), or hypointense (n = 1) on T1 but all were markedly hypointense on T2-weighted sequences. The contents of the cystic lesions in these seven patients ranged from pastelike to solid and had to be removed surgically. This study concludes that the observed T1- and T2-weighted signal intensities can predict the relative viscosity and the composition of intracranial cystic contents. This information is found to be quite useful in planning surgery and using appropriate instrumentation in the management of intracranial cystic masses.