Ordinal regression problem arises in situations where examples are rated in an ordinal scale. In practice, labeled ordinal data are difficult to obtain while unlabeled ordinal data are available in abundance. Designing a probabilistic semi-supervised classifier to perform ordinal regression is challenging. In this work, we propose a novel approach for semi-supervised ordinal regression using Gaussian Processes (GP). It uses the expectation-propagation approximation idea, widely used for GP ordinal regression problem. The proposed approach makes use of unlabeled data in addition to the labeled data to learn a model by matching ordinal label distributions approximately between labeled and unlabeled data. The resulting mixed integer programming problem, involving model parameters (real-valued) and ordinal labels (integers) as variables, is solved efficiently using a sequence of alternating optimization steps. Experimental results on synthetic, bench-mark and real-world data sets demonstrate that the proposed GP based approach makes effective use of the unlabeled data to give better generalization performance (on the absolute error metric, in particular) than the supervised approach. Thus, it is a useful approach for probabilistic semi-supervised ordinal regression problem. © 2013 Springer-Verlag.