Objectives The aim of this study was to assess the impact of onychectomy (declawing) upon subsequent development of back pain and unwanted behavior in cohorts of treated and control cats housed in two different locations. Methods This was a retrospective cohort study. In total, there was 137 declawed and 137 non-declawed cats, of which 176 were owned cats (88 declawed, 88 non-declawed) and 98 were shelter cats (49 declawed and 49 non-declawed). All cats were physically examined for signs of pain and barbering. The previous 2 years of medical history were reviewed for documented unwanted behavior such as inappropriate elimination and biting with minimal provocation and aggression. All declawed cats were radiographed for distal limb abnormalities, including P3 (third phalanx) bone fragments. The associations of declaw surgery with the outcomes of interest were examined using χ2 analysis, two sample t-tests and manual, backwards, stepwise logistic regression. Results Significant increases in the odds of back pain (odds ratio [OR] 2.9), periuria/perichezia (OR 7.2), biting (OR 4.5) and barbering (OR 3.06) occurred in declawed compared with control cats. Of the 137 declawed cats, 86 (63%) showed radiographic evidence of residual P3 fragments. The odds of back pain (OR 2.66), periuria/perichezia (OR 2.52) and aggression (OR 8.9) were significantly increased in declawed cats with retained P3 fragments compared with those declawed cats without. Optimal surgical technique, with removal of P3 in its entirety, was associated with fewer adverse outcomes and lower odds of these outcomes, but operated animals remained at increased odds of biting (OR 3.0) and undesirable habits of elimination (OR 4.0) compared with non-surgical controls. Conclusions and relevance Declawing cats increases the risk of unwanted behaviors and may increase risk for developing back pain. Evidence of inadequate surgical technique was common in the study population. Among declawed cats, retained P3 fragments further increased the risk of developing back pain and adverse behaviors. The use of optimal surgical technique does not eliminate the risk of adverse behavior subsequent to onychectomy.