The present invention relates generally to a method of diagnosing and/or monitoring the development or progress of chronic fatigue syndrome. More particularly, the present invention relates to a method of diagnosing and/or monitoring the development or progress of chronic fatigue syndrome by analysis of activin ß? expression levels in a subject mammal or in a biological sample derived from said mammal. This may be achieved by screening for activin ßB in either monomeric form or in dimeric form. Still further, the ratio of the dimeric form of activin ß? relative to follistatin and activin A levels also provides a useful diagnostic indicator. In a related aspect there is provided a method for the treatment of chronic fatigue syndrome by dovvnregulating the functional level of activin B.
