Summary In a material consisting of 100 patients, the marginal erosions demonstrated with Nørgaard's technique were one of the first objective signs, and, in several cases, they appeared even several months before the serological tests were positive. It must be clear that Nørgaard's definition must be strictly followed and therefore not only localized increased osteoporosis, but also marginal erosions with irregularity of the border of the bone must be seen, if the finding should be considered positive. The marginal osteoporosis must be due to an influence of the swollen capsule. They may disappear after a successful gold treatment but on the other hand they may persist through many years without progression even if, for instance, the metacarpal heads are destructed. The diagnostic value of the marginal osteoporosis and erosions for the support and establishment of an early clinical diagnosis is obvious. However, with a view to the differential diagnosis against gout and simple osteoarthrosis of the finger joint, it is less useful because somewhat similar marginal erosions of a somewhat similar character are often found in these conditions. Still, for a close observation there is a difference so that the bottom and the border of the marginal erosion is less irregular in gout and osteoarthrosis than in rheumatoid arthritis.