Rectal carcinoid <10 mm can be completely removed by endoscopic procedure. However, a few authors reported that small rectal carcinoids have the potential to metastasize. Therefore, histological parameters predicting their metastatic potential are necessary. The aim of the present study was to evaluate the usefulness of Ki-67 positive/negative area ratio (Ki-67 ratio) using computer-assisted image analysis. A total of 43 patients with rectal carcinoids were retrospectively investigated. Twenty-eight patients underwent endoscopic resection and 15 patients underwent surgery. Nine of the 43 patients presented with metastatic disease. The Ki-67 ratio of all specimens was measured. Finally, the Ki-67 ratio was compared with other macroscopic or microscopic parameters for predicting lymph node or distant metastatic disease. The mean Ki-67 ratio of the metastatic and non-metastatic group was 3.9 and 1.0%, respectively (P < 0.01). The Ki-67 ratio was effective because of its high sensitivity (88.9%) and specificity (82.4%). Ki-67 ratio was superior to other microscopic parameters such as variation in nuclear size and vessel permeation. Ki-67 ratio was as effective as the other macroscopic parameters of tumor size and central depression. Based on these results it is suggested that Ki-67 is a reliable microscopic marker to predict metastatic potential of rectal carcinoids.