StreamWritter doesn't close in Finally

Hi all,

In global.asax, in Application_BeginRequest, I have a condition - under this condition I execute SQL stored procedures, I append information and exceptions to a log file and I send email. This is the structure of all:

if (my conditions, including a GET from a local scheduled Windows task to run in the morning)
{
    logFile = HttpContext.Current.Server.MapPath("my path here containing the DateTime.Now.ToString bla bla.txt");
    StreamWritter sw = new StreamWriter(logFile, true);   //so the file is created if it doesn't exist (at the beginning of each month), otherwise appended
    sw.WriteLine("{0} : Begin running the job", currentDate.ToString());
    try
   {
      other try-catch-finally, ifs etc
    }
   catch (Excep ...