Objectives In this paper, we have analysed blue coloured skin over involved joints as a one of the clinical features of the psoriatic arthritis that is helpful for simpler and earlier reaching the diagnosis.

Methods Clinical material consisted of 650 consecutively hospitalised and outdepartment patients with the diagnosis of PsA and 1265 patients with diagnosis of RA in period between 1975 and 1998 year. Special attention was attended to inspection of the skin over involved joint including skin appearance, colour and size of the involved area and its changes in different conditions of the weather and temperature.

Results We have found blue coloured skin over involved joints in 598 patients with PsA (92%) and in 63 patients with RA (5%) (N2 joints and digits in case of sausage digits. In less involved joints blue colour develops in minor area, usually on central dorsal part of the sin over the joint. Blue coloured skin was better visible in the cold winter days while in the summer season and during the remission of the disease as well as at room temperature, blue colour is less marked or even disappears.

Conclusion This sign is reliable, so it should be very useful in PsA diagnostics.