Isthmic spondylolisthesis of the lumbosacral spine is a common radiologic finding in patients with low back pain, but its prevalence in unselected populations is poorly known. We examined X-ray pictures of the lumbosacral region of 1147 randomly selected 45-64-yr-old subjects living in eastern or western Finland. Of them, 69 subjects were found to have isthmic spondylolisthesis. The finding was more common in men (7.7%) than in women (4.6%; p = 0.026). The prevalence of spondylolisthesis was not age-related; it was higher in women with an occupation causing a high back load than in those with a low load occupation (p = 0.049), but this difference was not seen in men. The prevalence of spondylolisthesis tended to be higher in women with multiple pregnancies than in those with less than or equal to 2 pregnancies (p = 0.074). Women from eastern Finland had a higher prevalence of spondylolisthesis (7.2%) than those from western Finland (2.6%; p = 0.007), whereas no such difference was seen in men. The difference in women may partly be explained by factors causing loading of the low back, i.e. heavier work and a higher number of pregnancies.