The textile industry, known for its significant contribution to global greenhouse gas emissions, is increasingly active in exploring techniques and technologies to improve its environmental performance. The main tool to calculate environmental impacts is the Life Cycle Assessment (LCA) methodology, which is standardized and internationally recognized. Specific guidelines for the impact calculation of textile products are under development (Product Environmental Footprint Category Rules (PEFCRs) for the category of Apparel and Footwear). In this context, this study contributes to the knowledge in the textile sector through the development of a cradle-to-gate LCA of a woolen undershirt produced in Italy. This study shares robust and recent (2021) primary data for the phases of weaving, cutting, and sewing, obtained from an Italian company. Data from previous studies of the authors, as well as secondary data, are also used to complete the inventory. A further analysis is developed to include the use phase as well. The impact on climate change of one undershirt results in 11.7 kg CO2 eq, primarily due to the farming phase of sheep, which accounts for 88% of the total emissions. The impact on climate change of energy used in the wool transformation process has a relatively low impact (11%), also thanks to the partial use of electricity produced by photovoltaic panels, while materials (e.g., chemicals) and transportation have negligible contributions. The farming phase, despite relying on secondary data, is identified as the primary contributor for most of the other indicators. Additionally, it has been found that user habits play a key role in the impact related to one wearing of the undershirt. The findings suggest that further work is necessary in the textile sector and emphasize (i) the need for guidelines, enabling the inclusion of the use phase without compromising the comparability between different LCAs of similar textile products; (ii) the need for improved traceability practices in the textile sector, to enhance inventory data collection on the raw material production (wool fibers in the case under analysis).