By truncating, do you mean using the_excerpt() rather than the_content()? Or do you mean using the_content() with <!--more--> tags ?.
In either case, you can alter how a post behaves by using the following:
<?php
if (have_posts()) :
while (have_posts()) : the_post();
//We check that we are this is the first post in the loop AND
//that we are on page one.
//(Probably unnecessary) but to be safe you can uncomment the next line
//global $wp_query;
if(!is_paged() && $wp_query->current_post==0){
//First post of first page, show full content using the_content();
//This will display up to the first <!--more--> tag - not clear if you are using this.
}else{
//Otherwise display the post as normal / using the_excerpt();
}
endwhile;
endif;
?>