A hand-operated inflator device for inflating the expander on a balloon-type catheter. It comprises a molded plastic housing for retaining a syringe tube therein for containing an inflation fluid. The housing also includes an internally threaded surface. A syringe plunger having an internal longitudinal bore and a longitudinal slot formed through the wall thereof has a piston mounted on the distal end for insertion into the syringe body. Fitted into the longitudinal slot is an externally threaded member which can be raised and lowered in a radial direction by manipulation of a manuable actuable means positioned in the longitudinal bore of the plunger. When the externally threaded member is raised, its teeth mesh with the teeth on the internally threaded annular surface of the housing, but when lowered, the threaded surfaces are disengaged. When the two are engaged, the piston is advanced in the syringe tube by rotating the plunger. When the two are disengaged, the plunger may be advanced by longitudinal force applied to the end of the plunger.