Purpose: To report an alternative method for the diagnosis of atypical choroidal malignant melanoma—ThinPrep® sampling of the vitreous cavity through 25-gauge vitrectomy. Method: Case report. Results: A 79-year-old gentleman presented with a progressive decline in visual acuity in his left eye. A clinical diagnosis of large choroidal malignant melanoma was made. Given the history of trauma and the possibility of enucleation, the patient wished to have histopathological confirmation. The patient underwent 25-gauge vitrectomy. However, he developed a choroidal hemorrhage during the procedure and the surgeon elected not to perform transretinal biopsy. Instead a sample of the vitreous was evaluated by ThinPrep® analysis, and a diagnosis of choroidal malignant melanoma was made. Conclusions: Cytological evaluation of vitreous substance with ThinPrep® may provide an alterative method for a diagnosis in select cases in which FNA is not possible or desirable. This technique is straightforward and minimizes the risk of retinal detachment or hemorrhage.