The objective of this study is to develop a model for measuring adherence to lean practices for automotive part suppliers and to assess the relationship between the firm performance and the adoption of lean principles. This model has been developed by utilising the lean practices defined in the literature. The proposed model has been successfully applied to a large number of automotive part suppliers, and has been tested for its statistical validity. To test the proposed model, data were obtained from 207 automotive part suppliers by using the Computer-aided Telephone Interview method. Validity and reliability tests of the developed model of leanness are realised through exploratory and confirmatory factor analyses. Correlation analyses are used to examine the relationships between the lean applications and company performance and size. Analysis of Variance (ANOVA) is applied to reveal whether there are differences in the use of lean techniques across different company categories. The study shows that, in general, Turkish automotive part suppliers are performing their internal production in compliance with the lean manufacturing principles. Furthermore, it is found that there is a significant relationship between adaptation of lean principles by the supplier firms and their performance. The proposed model of leanness can be used by companies in the automotive industry to measure their leanness levels.