<p>Previous research on linguistic complexity features in research articles (RAs) has mainly focused on synchronic comparisons across disciplines or expertise levels. Relatively little research has been conducted from a diachronic perspective. The aim of the present study was to examine diachronic changes and disciplinary variations in the lexical complexity of RAs. A diachronic corpus consisting of 960 journal articles published between 1970 and 2020 in economics and medicine was compiled for analysis. Nine measures incorporated in Lu’s (2012) Lexical Complexity Analyzer were adopted to operationalize three dimensions of lexical complexity for all the texts. Results showed an increasing trend over the 50&#xa0;years in the lexical density, variation, and sophistication of RAs in the two disciplines. More specifically, RAs in both disciplines have become more lexically complex over time, producing an increasing proportion of lexical words, a larger number of sophisticated words, and a wider diversity of word types. Results also showed that the two disciplines varied significantly in the complexity measures, with medicine generally exhibiting a higher level of lexical complexity during the examined period. These observed changes and variations could be interpreted as a result of shifting research paradigms and practices in the two disciplines. The findings of this study would facilitate a further understanding of linguistic changes occurring in disciplinary writing.</p>