Study aims were to evaluate if medial patellar luxation clinical grades changed with oestrus status, and to determine interobserver agreement for different classification methods for grading patellar luxation. Intact female dogs were recruited for grading by 3 independent observers on 2 occasions (pro-oestrus/oestrus and dioestrus/anoestrus) using a published grading system. Observers were blinded to oestrus status. Oestrus status was confirmed by vaginal cytology. Observer agreement was determined using Fleiss' multirater kappa on the original grading scores, simplification to the Norwegian Kennel Club system, and further simplification to a binary suitable/not-suitable for breeding system. The exact McNemar's test was used to assess the effect of oestrus on classification. Of 70 dogs recruited, 53 underwent paired observations. Interobserver agreement was considered moderate to very good for the study sub-groups, with overall kappa values of 0.68 (95% CI 0.63-0.72), 0.79 (0.73-0.84) and 0.92 (0.85-0.99), and percentage agreements of 65%, 81% and 94%, for the original, simplified and binary classifications. Oestrus status did not have a significant effect on classification of breeding suitability. Clinicians and owners should not be concerned about the timing of patellar luxation grading in relation to oestrus. Experienced observers show good or very good agreement using the Norwegian Kennel Club and binary categorisations.