In documents using the book or report class the first page of a chapter will have the page style plain, the page number will appear at the bottom of the page. If you want to remove the number you could write \thispagestyle{empty} on each of those pages, or you could force the plain page style to be empty:

\makeatletter
\let\ps@plain\ps@empty
\makeatother

If you use the headings or fancy pagestyle and you want that pagestyle for opening pages of chapters too, you can write similar for instance

\let\ps@plain\ps@fancy

and all plain pages will look like the fancy style.

This topic was discussed in the LaTeX Community Forum on mrunix.de and on CQF.info.