Percutaneous left ventricular catheterisation with a 4F (O.D. 1.33 mm) catheter-tip pressure transducer is described. The transducer is small enough so that it can be introduced through a short 4F arterial insertion sheath or a Brockenbrough Trans-septal catheter. Experience in 35 patients suggests that this ultraminiature catheter mounted pressure transducer is a valuable addition to the cardiac catheterisation laboratory.