The aim of the work was to find an answer to the question of how the changes in the milk production sector in Poland, following the marketization of the economy, influenced the country’s food security in the context of food self-sufficiency. The paper uses cow milk balances developed by the Central Statistical Office and data concerning the distribution of domestic milk production from the Institute of Agricultural and Food Economics National Research Institute. The study used a comparative analysis over time, indicators as well as descriptive and graphic methods. The indicators used concerned milk market output, food self-sufficiency and the milk balance structure. During the first few years of transformation, there was a sharp drop in domestic milk production. Since 2004, there has been, with minor exceptions, a systematic increase in production with a previous general fall in production volume. These processes were accompanied by changes resulting from the transformation of the economy in the milk production sector. These were associated with a drastic reduction, since 1990, of the number of farms involved in milk production and the decline in the number of dairy cows, albeit with a simultaneous increase in milk yield. The milk market output index increased. In 2015, resource in the milk balance returned to the level of 1991, i.e. over 14.5 bln liters. In 2017, this figure amounted to 15 bln liters. Domestic milk production is the main source of resource, though the volume of imports is significantly increasing. Despite dynamic growth of exports, milk is mainly used for domestic consumption. From 1990 onwards, Poland has significantly improved its self-sufficiency in milk production. Changes in milk production have significantly influenced food security in the country.