OBJECTIVE: The first choice treatment in stress urinary incontinence (SUI) is rehabilitation of the pelvic floor in order to improve muscle strength. However, no entirely reliable instruments for quantifying pelvic floor muscle (PFM) strength are currently available. Our aim was to test the inter-rater reliability of the Modified Oxford Scale (MOS) used as the gold standard for measuring PFM strength.METHODS: Test-retest reliability study. One hundred and twenty-two women with SUI and thirty continent women were recruited. Patients were excluded if they had a history consistent with pelvic organ prolapse, pregnancy, previous urogynaecological surgery, or neurological conditions. Bidigital palpation quantified by the MOS was carried out by three independent examiners.Each subject answered a two-part questionnaire: demographic and clinical characteristics and the International Consultation on Incontinence Questionnaire-Short Form (ICIQ-SF). The Cohen’s Kappa index with quadratic (Kw) weighting was used to assess the inter-rater agreement.RESULTS: 122 incontinent and 30 continent women were included. MOS ranged between 0 and 2 in 72.2% incontinent and 19.5% of continent women.The degree of agreement was substantial in the group of incontinent women (Kw between 0.73 and 0.81) and moderate in the group of continent women (Kw between 0.55 and 0.72).CONCLUSIONS: Our results suggest that the inter-observer reliability of MOS is improbable, especially in continent women.