To determine the prognostic significance of CT‐determined cachexia scores (CSs) in 127 consecutive male small cell lung cancer (SCLC) patients, cross‐sectional areas of muscle and fat tissues at the third lumbar vertebra (L3) were retrospectively measured on baseline CT images. CSs were determined based on the presence of sarcopenia and/or adipopenia. According to the presence of sarcopenia (L3 muscle index <55 cm2/m2, 86.8%) and adipopenia (L3 fat index <22 cm2/m2, 11.8%), CSs were defined as follows: CS2 (sarcopenia and adipopenia, 11.8%), CS1 (sarcopenia only, 74.8%) and CS0 (13.4%). CS2 was significantly related to lower body mass index (p < .001) and poor performance status (p = .002), and patients with CS2 had shorter OS than patients with CS1 or CS0 (median OS, 5.0 months vs. 8.9 months vs. 18.3 months; p = .007). Multivariable analysis revealed that CS was an independent prognostic factor of poor survival (HR, 1.99 for CS1 and 2.59 for CS2, p = .036 and .023, CS0 as a reference), along with extensive stage (p < .001), supportive care only (p < .001) and an elevated lactate dehydrogenase (p = .005). CT‐determined CSs, based on the presence of sarcopenia and/or adipopenia, could be used to predict prognosis in male SCLC.