Analysis of the reduction in sizes of particles after chewing has been a standard method for assessing masticatory efficiency for over 40 years, but the sizing of particles with sieves is very time consuming. A rapid simple method of measuring chewed almond particle sizes by an optical scanning method has been developed and is described. The aim of this study is to validate the accuracy, repeatability and speed of the application of an optical scanning system in measuring chewed particles. To determine the accuracy, paper circles were measured and compared with the travelling microscope measurements. The effect of orientation of the images were assessed by measuring three irregular shaped pieces of paper and three different sized pieces of chewed almonds. Measurements were taken singly at 18 degrees increments from 0-180 degrees orientations with each piece of paper. To assess the repeatability of measuring chewed particles, the particles from a single masticated almond washed with water and absolute alcohol were spread, separated and measured by the optical scanning system. The samples were then remeasured 10 times. The reproducibility of the method was investigated using three repeated masticatory tests for each subject in a group of 13 young dentate subjects. The speed of spreading the particles and of measurement was assessed. The absolute error range for a mean area of 31 mm2 was 2.85% to 7.32% with a mean of 3.86%. The relative accuracy of measurement was higher for larger particles but in no case was the standard deviation > 0.4 mm2.(ABSTRACT TRUNCATED AT 250 WORDS)