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Feline Odontoclastic Resorptive Lesions (usually abbreviated as FORL, formerly ,neck lesions") are lacunar defects in the hard tooth tissue. The disorder plays a significant role because of progression in the cat population. Long described in the literature, and in spite of intensive research, its aetiology still poses many questions today.Various therapeutic approaches have shown that because of the high degree of pain in the process, only extraction of the affected eeth will eliminate the patient"s suffering. Attempts to preserve the teeth have so far not been satisfactorily successful. For this reason, investigations to explain the pathogenesis and aetiology are useful in better understanding the disorder.Therefore cats that had died or been put down were examined for the presence of FORL in a Berlin veterinary practice. 45 animals showing both teeth with visible resorptive lacunae and clinically healthy teeth underwent the examination. The mouth cavity findings for the animals were recorded. 80 clinically healthy teeth in 61 jaw segments were removed, fixed and later examined histologically and histochemically. Finally the body of the animal was dissected. In the pathologicalanatomical examination in 14 animals particular attention was paid to the tonsils and other lymphatic organs of the neck region.The results on gingivitis symptoms and tartar affection, as well as the frequency distribution and localisation of FORL and tooth loss confirm the literature published so far.It was the first time clinically healthy teeth from FORL sufferers were examined. The central question of the histological examination was:Is the clinically healthy tooth in the tooth socket also histologically normal?".During the microscopic examination all the tooth sections (crown, neck and roots) and the periodontium were evaluated and anomalies were classified under the phases, postulated by ROES (1996) (resorption phase, transformation phase, formation of osteoid and ossification phase). Predilection points in the various tooth sections were analysed for the individual phases. FORL-related changes were found in the neck in 25 (31.3 %, n = 80) teeth, in the roots in 63 teeth (78.8 %, n = 80) and in the periodontium in all 80 (100.0 %, n = 80) teeth.The hypothesis on pathogenesis put forward by ROES (1996) was confirmed. The updated definition of FORL is the following: FORL in cats is a disorder of the periodontim which begins exogenously, usually in the root area, is characterised by the cellular deterioration of the hard tooth tissue, and which initially odontoclasts and later also plasma cells play a central role.The idea of filling therapy must be discareded as it brings no hope of cure. FORL is not an visible, solid process. As a rule those parts of the tooth are affected which are inaccessible to clinical examination, and the periodontium is always affected. These are localisations unsuited to conservations measures. It is advisable to extract all teeth which show cavities at the cementoenamel junction, but refrain from comprehensive extraction due to the continuing functionality of clinically healthy teeth.Animal dissections have demonstrated that some of the animals show a main disorder which can be classified as immunosuppressive. As well as malignant oncological disorders, leucosis with clinical symptoms should be mentioned here. The suspicion arises that an immune suppression could e one of the triggering factors of this disorder with multiple causes.