Background: Muscle wasting is associated with increased risk for mortality. There is no agreed universal definition for muscle wasting (sarcopenia), and we wished to determine whether using different criteria altered the prevalence in patients treated by peritoneal dialysis. Methods: We measured lean body and appendicular lean mass indices in 325 outpatients by dual-energy x-ray absorptiometry, comparing muscle mass with that used to define muscle wasting (sarcopenia) by various clinical guideline publications. Results: Lean body and appendicular lean mass indices did not differ by sex: female, 17.7 ± 4.6 kg/m2; male, 17.4 ± 4.3; female, 6.9 (5.6–8.5) kg/m2; male, 6.7 (5.3–8.3), respectively. Depending on the criteria, the prevalence of muscle wasting varied from 2.2%–31.3% for women and 25.1%–75.6% for men. Male patients were older (58.3 ± 16 vs 53.4 ± 15.7 years). Criteria based on cutoffs derived from young healthy patients gave the higher prevalence rates. The prevalence of muscle wasting was not associated with dialysis adequacy, estimated protein intake, duration of dialysis treatment, comorbidity, diabetes, or ethnicity. The prevalence of sarcopenic obesity was low (<5% females, 7% males). Conclusion: We found that the prevalence varied markedly depending on the cutoff criteria used to define muscle wasting. Very few patients had sarcopenic obesity. The higher prevalence for males requires further study but was not associated with dialysis treatment. Our study highlights the need for agreed criteria to define pathologic muscle wasting from that which is age associated to allow for interventional screening programs.