Lettuce is a short-season vegetable crop with low-temperature requirements, making it suitable for winter cultivation in unheated protected spaces, during which favorable conditions for the occurrence of certain plant diseases prevail. To grow it successfully, it is necessary to select tolerant or resistant varieties/hybrids planted at an appropriate plant density. Therefore, the aim of this research was to determine lettuce yield at different planting densities and monitor the intensity of plant diseases during winter cultivation. The 'Posavka' variety was grown, and the tested planting spacings were 35x30 cm, 35x35 cm, and 35x40 cm. Throughout the cultivation, temperature and relative humidity conditions in the greenhouse were monitored daily. A visual inspection of plants was conducted weekly to assess the intensity of plant disease occurrence and determine the types of plant diseases. The number of infected plants per variant and the extent of damage were recorded. Biomass and marketable head mass, yield, and marketable lettuce yield were determined during harvest.The highest number of plants infected with gray mold and downy mildew was recorded in the plot with the lowest planting density, i.e., a plant spacing of 35x40 cm. On the other hand, the highest plant mortality after planting was observed in the planting spacing of 35x30 cm. The highest yield was obtained in the plot with a plant spacing of 35x30 cm. From the above, we conclude that the plot with the highest planting density resulted in the highest yield and marketable mass, but also in the highest number of plant deaths after planting."