Changes in the progression of disability is still the main variable measured in clinical trials involving patients with multiple sclerosis (EM). The amplified scale of the state of disability (EEDA) of Kurtzke continues to be the most widely used scale. One of the most important limitations of the EEDA is interobserver variability. The object of our study was to find the interobserver variability between members of our EM unit in patients with EEDA between 0 and 3.5 when applying functional systems (SF) and EEDA. Seven patients with recurrent remittent EM (EEDA 0-3.5), participating in a multicentric trial with natural beta-interferon, were assessed monthly for 12 months by two of four neurologists. The SF were assessed and the EEDA applied separately and without knowing the former. 80 paired examinations were made. Complete concordance of SF was only seen when a variation of 2 points was allowed, while it was very high for a variation of 0 and 1 points. Complete concordance for EEDA was seen in 36.6% of the cases and only in 11% was there discordance of 1 or 1.5 points. The average variability of EDSS was 0.39. There was greater variability when the two scores were at different levels of incapacity.