session_start() and headers

Are there any headers sent to the browser in session_start()? I ask because I have a script that sends data as a PDF document to the browser. Before I'm outputting the data, I'm sending the header: header( "Content-Type: application/pdf" ); to tell the browser that it's a PDF and to load as such. If I don't use session_start(), everything works as expected. The PDF comes up and all is right with the world. However, as soon as I uncomment session_start(), the PDF doesn't load correctly and in fact, I get a save dialogue that doesn't work right (ie, I get an error when I click any of the buttons). So, is there a header getting sent by session_start()? Is there any way I can turn that off? Has anyone found a workaround for this? Any help would be greatly appreciated! Chris