PURPOSE: To evaluate the association between being underweight and shockwave lithotripsy outcomes. METHODS: This retrospective two-centre cohort study conducted in Japan involved 597 patients diagnosed with a single urinary tract calculus based on computed tomography and who underwent shockwave lithotripsy between 2006 and 2016. We divided the patients into four groups based on their body mass index (underweight, ≤  18.4; normal weight, 18.5-24.9; overweight, 25-29.9; obese,  ≥  30 kg/m2). We performed multivariable logistic regression analysis and estimated the odds ratio for success of single-session shockwave lithotripsy. RESULTS: Of the 597 patients, 25 (4.2%) were underweight and 34 (5.7%) were obese. After adjusting for age, sex, calculus localisation, maximum stone length, mean stone density, and skin-to-stone distance, being underweight showed a significantly negative association with success of single-session shockwave lithotripsy (odds ratio 0.25, 95% confidence interval 0.09-0.69) compared to being normal weight. CONCLUSIONS: This study showed the negative impact of being underweight on the outcomes of shockwave lithotripsy in patients with upper urinary tract calculi. This finding provides a novel viewpoint regarding the body mass index and should aid improved treatment selection for patients with upper urinary tract calculi.