Stereotactic aspiration is a valuable surgical alternative for colloid cysts, but due to cyst heterogeneity, it is not uniformly successful as the sole treatment in all patients. Since 1981, we performed CT-guided stereotactic aspiration as the initial procedure in 25 patients with colloid cysts. We retrospectively reviewed our experience in the first 22 patients and found that preoperative CT imaging studies accurately determined size and predicted cyst viscosity. The preoperative CT appearance of a hypodense or isodense cyst correlated favorably with successful aspiration. Preoperative MRI provided excellent anatomic definition of the cyst and its relationship to other structures, but failed to correlate successful aspiration. Unsuccessful aspiration was related to the high viscosity of the intracystic material or deviation of the cyst away from the aspiration probe. These factors were used prospectively in 3 additional patients to accurately predict success of cyst aspiration.