A catheter insertion sheath assembly (100) including a tubular sheath body (106) having a proximal end (102), a distal end (104), and a hollow passage (107) extending between the proximal end and the distal end. A handle portion (110,114) is disposed at the proximal end of the tubular sheath. A clamp (120;300;400) extends from the handle portion for releasably closing the sheath prior to insertion of a catheter thereinto and includes a relatively movable portion (130) or portions (310,310′;410,410′) that is or are movable between an open position and a closed position, wherein, when the clamp is in the open position, fluid may flow through the hollow passage between the proximal end and the distal end and when the clamp is in the closed position, fluid flow is restricted through the hollow passage between the proximal end and the distal end. A method includes inserting a catheter into the sheath upon the clamp's movable portion(s) being moved to an open position.