The rationale of this study was to investigate the performance of high-resolution CT (HRCT) versus ¹⁸ F-FDG PET/CT for the diagnosis of pulmonary lymphangitic carcinomatosis (PLC). Methods: In this retrospective institution-approved study, 94 patients addressed for initial staging of lung cancer with suspicion of PLC were included. Using double-blind analysis, we assessed the presence of signs favoring PLC on HRCT (smooth or nodular septal lines, subpleural nodularity, peribronchovascular thickening, satellite nodules, lymph node enlargement, and pleural effusion). ¹⁸ F-FDG PET/CT images were reviewed to qualitatively evaluate peritumoral uptake and to quantify tracer uptake in the tumoral and peritumoral areas. Histology performed on surgical specimens served as the gold standard for all patients. Results: Among 94 included patients, 73% (69/94) had histologically confirmed PLC. Peribronchovascular thickening, lymph node involvement, and increased peritumoral uptake were more often present in patients with PLC (P < 0.009). Metabolic variables, including tumor SUV _max , SUV _mean , metabolic tumor volume, and total lesion glycolysis, as well as peritumoral SUV _max , SUV _mean , and their respective ratios to background, were significantly higher in the PLC group than in the non-PLC group (P ≤ 0.0039). Sensitivity, specificity, and area under the receiver-operating-characteristic curve for peribronchovascular thickening (69%, 83%, and 0.76, respectively; 95% confidence interval [95%CI], 0.67-0.85) and increased peritumoral uptake (94%, 84%, and 0.89, respectively; 95%CI, 0.81-0.97) were similar (P = 0.054). For detecting PLC, sensitivity, specificity, and area under the receiver-operating-characteristic curve were significantly higher, at 97%, 92%, and 0.98, respectively (95%CI, 0.96-1.00), for peritumoral SUV _max and 94%, 88%, and 0.96, respectively (95%CI, 0.92-1.00), for peritumoral SUV _mean (all P ≤ 0.025). Conclusion: Qualitative evaluation of ¹⁸ F-FDG PET/CT and HRCT perform similarly for the diagnosis of PLC, with both being outperformed by ¹⁸ F-FDG PET/CT quantitative parameters.