A low profile adaptor for use with a medical catheter. In one embodiment, the adaptor comprises a lower connector portion, an upper connector portion and a valve. The lower connector portion comprises a tubular portion and an annular portion, the annular portion radially surrounding the tubular portion along an intermediate length thereof. A helical thread is formed on the inside surface of the tubular portion. A circular lip projects upwardly a short distance from the top surface of the annular portion. The upper connector portion comprises a tubular section adapted for insertion into the tubular portion of the lower connector portion. An external helical thread is formed on the tubular section of the upper connector portion for mating engagement with the thread on the interior of the tubular portion of the lower connector portion. The upper connector portion also includes an annular base. The top of the base is shaped to include a cavity for receiving the valve, the cavity being in fluid communication with the tubular section. A double-walled circular lip is formed on the bottom of the base, the double-walled lip being sized and shaped to define a groove adapted to matingly receive the lip on the upper connector portion. In use, the lower connector portion is inserted into the end of a catheter, and the tubular section of the upper connector portion is screwed into the tubular portion of the lower connector, with the catheter being ensnared between the lips of the upper and lower connector portions.