We describe a new unit of the Jena retinal camera for automatic measurement of retinal vessel diameter. The brightness profile perpendicular to the vessel is registered by CCD line sensor and the signal is transmitted to the computer. An adaptive computer program recognizes the edges of vessels and estimates the vessel diameters automatically. Applying several measurements per vessel, the intraindividual standard deviation may be smaller than 3%. By using the same objective measuring edge criterion, the measuring results are comparable between different observers and methods. Error sources are discussed. Measured vessel reactions to oxygen breathing and reduced perfusion pressure are presented to demonstrate the efficiency of this new method.