The incidence and age of onset of daily knee pain for more than 3 months each year were determined in the various topographical varieties of knee osteoarthrosis (404 knees, 258 patients: 195 women and 63 men). Statistical calculation revealed no significant difference between the incidence of forms without chronic pain in the various topographical types of knee osteoarthrosis. In women, the mean age of onset of chronic pain in lateral femoro-patellar osteoarthrosis was 56.6 +/- 12, in medial femora-tibial osteoarthrosis was 62.7 +/- 12, in the combination of the two 65.2 +/- 10 and in lateral femoro-tibial osteoarthrosis was 69.2 +/- 10. In men, this age in lateral femoro-patellar osteoarthrosis was 60.5 +/- 10 and in medial femoro-tibial osteoarthrosis was 64 +/- 10. Thus lateral femoro-patellar osteoarthrosis caused chronic pain an average of 5 years before medial femoro-tibial osteoarthrosis, and the latter an average of 7 years before lateral femoro-tibial osteoarthrosis.