Lean practices appear to have aided organisations in improving their operations, especially manufacturing companies. This paper reports on an in-depth study comparing Thai medium-size manufacturing companies who use Lean with those which have not yet adopted Lean. In this paper the word “Lean” is used to refer to “lean practices” as well as “lean strategies”. Lean methods include just-in-time (JIT), total productive maintenance (TPM), automation, value-stream mapping (VSM), Kaizen, material requirement planning (MRP), Kanban, 5S and waste elimination. Primary data were obtained through questionnaires which were analysed using a number of statistical processes including factor analysis (CFA) and structural equation modelling (SEM). Some 230 medium-sized manufacturing companies were the focal target of this study. Three latent variables, Lean (exogenous variable), product customisation (endogenous) and operational performance (endogenous) are formed in this study. There were 24 parameters and the findings provide further evidence regarding the effects which Lean practices have on product customisation and operational performance of the companies, for example. The methods for analyzing the data favour the proposed structural equation model. The results show that Lean strategies are linked to improved operational performance in the firms which practice them. Management teams in Thai medium-size manufacturing companies which have resisted Lean are encouraged to adopt Lean in order to improve operational performance, particularly in the present economic climate.