This study examines if informed trading is present in the index option market by analyzing the KOSPI 200 options, the most actively traded derivative product in the world. The spread decomposition model developed by Madhavan, Richardson, and Roomans (1997) is utilized and the adverse-selection cost component of the spread estimated by the model is then used as a proxy for the degree of informed trading. We find that adverse-selection costs constitute a nontrivial portion of the transaction costs in index options trading. Approximately one-third of the spread can be accounted for by information asymmetry costs. A further analysis indicates that adverse-selection costs are positively related with option delta. Our regression analysis shows that option-related variables are significantly associated with estimated information asymmetry costs, even when controlling for proxies for informed trading in the index futures market. Finally, we find the evidence that foreign investors are better informed compared to domestic investors and that domestic institutions have an edge in terms of information over domestic individuals.