The sensitivity of chest radiography for the early detection of mycetoma formation within fibrotic cavities is poor. The purpose of this study was to determine the predictive value of the secondary sign of lateral cavity wall thickening for the detection of a radiographically occult mycetoma. The chest radiographs and CT scans of 70 patients who had a total of 109 fibrotic cavities on CT were reviewed by two observers. Dimensions of the cavity, mycetoma, and cavity wall thickness on chest radiography and CT scans were recorded. Mycetomas were visible in 41 of 99 cavities on chest radiographs and in 61 of 109 cavities on CT. Using CT as the gold standard for detecting the presence of mycetomas, the sensitivity of chest radiography for the presence of a mycetoma was 62 % and the specificity 94 %, and the positive and negative predictive values were 93 and 66 %, respectively. On logistic regression analysis, lateral wall thickness on chest radiography was predictive of the presence of a mycetoma (p < 0.0005) independent of other radiographic features. In patients with chronic fibrocavitary disease on chest radiography, the presence of lateral wall thickening is highly suggestive of an underlying mycetoma.