Background Pulmonary nodule features have been used to differentiate benign from malignant nodules. Purpose To determine the frequency of solid noncalcified nodules attached to the costal pleura (CP-NCNs) at baseline low-dose CT and to identify key features of benignity. Materials and Methods A retrospective review was performed of baseline low-dose CT scans obtained in 8730 participants in the Mount Sinai Early Lung and Cardiac Action Program screening cohort between 1992 and 2019. Participants with one or more solid CP-NCNs between 3.0 mm and 30.0 mm in average diameter were included. For each CP-NCN, the size, location, shape (lentiform, oval, or semicircular [LOS]; triangular; polygonal; round; or irregular), margin (smooth or nonsmooth), and attachment to the costal pleura (broad or narrow) were documented. The manifestation of emphysema and fibrosis within a 10-mm radius of the CP-NCN was determined. Multivariable logistic regression analysis, with synthetic minority oversampling techniques, was used. Results The 569 eligible participants (average age, 62 years ± 9 [standard deviation]; 343 women) had 943 solid CP-NCNs, of which 934 (99.0%) were benign and nine (1.0%) were malignant. Multivariable analysis showed that five shapes could be consolidated into three (LOS and/or triangular, round and/or polygonal, and irregular shape); pleural attachment was not a significant independent predictor (odds ratio, 1.24; P = .70); and interaction terms of size with shape (odds ratio, 0.73; P = .005) and margin were significant (odds ratio, 0.80; P = .001). All 603 CP-NCNs less than 10.0 mm with LOS or triangular shapes and smooth margins were benign. Conclusion All baseline noncalcified solid nodules attached to the costal pleura less than 10.0 mm in average diameter with lentiform, oval, semicircular, or triangular shapes and smooth margins were benign; thus, for these nodules, an annual repeat scan in 1 year, rather than a more immediate work-up, is recommended. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Godoy in this issue.