How do I hide fields conditionally?
You need to set access to FALSE.

function MYTHEME_node_view($node, $view_mode, $langcode) {
  if(isset($node-&gt;content['field_members_only']) &amp;&amp; count($node-&gt;content['field_members_only']) &amp;&amp; $node-&gt;content['field_members_only']['#items']['0']['value']=="0") {
    $node-&gt;content['field_members_only']['#access'] = FALSE;
  }
}