The unechogenic space between the bone and the joint capsule in 280 metatarsophalangeal (MTP) joints and in 56 talocrural (TC) joints of 28 healthy adults was measured by ultrasonography. These normal values were compared with those of 40 arthritic MTP and of 15 arthritic TC joints. The unecogenic space in the MTP joints was more than 3 mm in 12 out of 280 healthy joints during dorsiflexion. In arthritic joints the space was more than 3 mm in 31 out of 40 joints, and the mean value was significantly higher than in the healthy joints. An unechogenic space could be demonstrated in all the arthritic but none of the healthy TC joints. An unechogenic space of more than 3 mm in a dorsiflexed MTP joint and demonstration of an unechogenic space in a TC joint are signs of intra-articular effusion or synovitis.