1. on the server-side, just populate both dropdown options exactly the same. 2. When the user selects an option from the first dropdown (using the onchange event) save the option selected in a variable, and then iterate through the options in the second dropdown and remove the one that matches the one selected in the first dropdown. 3. If the user changes the first dropdown again, in the second dropdown you will have to replace the option you removed in step 2, and then remove the newly selected option.