Computer processing of sacro-iliac (S-I) joint scintigrams represents a significant contribution to the diagnostic workup of this area. The S-I index is calculated as the ratio between the total number of counts in the region of interest placed on the S-I joint and an identical region placed on the sacrum. S-I indexes were determined in a group of 70 patients aged 13-77 years to establish the influence of age on normal index values. Our results indicate that S-I index values are age dependent. The study was performed 2 h post-injection but the possibility exists that this is not the optimal time interval.