A new method has been developed for determining the axial length of the human eye. This method is based on laser interferometry with partially coherent light emitted by a multimode semiconductor laser diode. The measurement procedure is as follows. The subject looks into the laser beam (wavelength approximately equal to 780 nm, power approximately equal to 100 microW), which appears as a red spot. Hence the laser beam acts as a fixation target and measuring beam. This beam passes a Fabry-Perot interferometer in front of the eye. Reflected beams are generated at the cornea and retina. Since the coherence length of the laser used (approximately equal to 25 microns) is less than the difference in path length of the two reflected beams, they will usually not interfere with each other. However, if the distance between the interferometer plates is equal to the optical length of the eye, an interference pattern can be observed. (This is due to a second illuminating beam, which is generated by the interferometer and delayed by two times the interferometer plate distance.) During measurement, the plates are displaced relative to each other until the interference pattern is observed. At this point the plate distance, which can readily be measured, is equal to the optical eye length. This method has several advantages in comparison with the usual ultrasonic technique: it is a non-contact method; no anesthesia is needed; the optical eye length is determined directly; no assumptions need be made about sound velocities in the different eye media. High longitudinal and transversal resolution is achieved (10 microns or better possible). The first measurements obtained with the optical method were compared with the results yielded by the ultrasound technique.(ABSTRACT TRUNCATED AT 250 WORDS)