I had a really weird issue with a ReportViewer. It would show the little green AJAX Loading… image but it wouldn’t show any data, just a blue background where the output should be. I knew the data was there because exporting to Excel or PDF showed the data just fine. So I eventually tried it in FireFox instead of IE and managed to see a few rows of data, but not much. That got me thinking that maybe it was a CSS or HTML issue — I’d set the height and width each to 100% because the default view was too small. So I set the height to 1000px and voila! It works. I wasted about 15 minutes of searching on this, so hopefully this will help save a few others some time.