A small fiber-optic sensor capable of measuring three parameters (temperature, pressure, and flow) is described. This sensor combines three destinct technologies and yet uses a single optical fiber. The temperature measurement is based on the photoluminescent decay time technology used commercially by Luxtron. Pressure is sensed by measuring the intensity of the return signal from a reflector or photoluminescent layer on the outer surface of a compressible dome-shaped optical element formed on the tip of the fiber. The local flow rate measurement is made by an optical heat balance technique similar to that used in more conventional hot wire (or hot film) anemometry. Studies conducted recently indicate that a submillimeter diameter sensor of this type can be built and should be suitable for in-vivo measurements in various applications such as during radio frequency (RF) or microwave heated angioplasty and in a continuous cardiac output monitoring.