Frustration in differentiating before operation among the various causes of midline neck masses stimulated us to review the clinical, operative, and pathologic findings in 176 children hospitalized for a midline cervical lump. The preoperative diagnosis was correct in 61% of these cases, the surgeon's postoperative diagnosis was correct in 83%, and the pathologist's diagnosis was correct in 98%. Only recurrence following prior excision and persistent sinus following drainage of a midline cyst were pathognomonic preoperative indicators of a thyroglossal duct cyst; these findings were present in only 6% of the cases. The age of the child, a history of inflammation or of fluctuation in size, the dimensions of the mass, and the relationship of the mass to the hyoid were useful in differentiating among groups of children with a thyroglossal cyst, an epidermoid cyst, or an enlarged perihyoid lymph node. The duration of the presence of the mass and its movement with swallowing were not helpful. Using the preoperatively recorded historical and physical findings in these children with a known cause for their midline neck mass and applying Bayes' theorem, one can only achieve a correct preoperative diagnosis with an 80% accuracy based on the best available data. Consequently, additional information obtained at surgical exploration is needed to establish a secure diagnosis and to ensure appropriate treatment.