Swine reproductive performance data gathered over a period of four years (2000 to 2003) from a swine production farm with 1,650 sows, located in Juiz de Fora, MG, Brazil, were analyzed with the objective of evaluating the effect of thermal environment on sow reproductive performance variables. For that region and for each month, Temperature-Humidity Index (THI) values were determined according to Thorn (1958), and the thermal zones were delimited as follows: thermal comfort zone (61<THJ<65), intermediate zone (65<TH1<69), and thermal stress zone (69>TH1<73). Among the analyzed variables, the following were influenced by the calculated TH1: Piglet mortality - more occurrences within the thermal stress zone; litter average weight at day 21 - lower average within the thermal stress zone; weaned piglets per sow - greater number within the intermediate zone; farrowing rate - higher value within the intermediate zone; average weight per weaned piglet - higher average within the thermal comfort zone; mummified fetuses - fewer occurrences within the thermal comfort zone; stillborns - fewer occurrences within the thermal comfort zone. Values ofTHl between 61 and 65 positively influenced sow reproductive performance. Thus, better animal performance and corresponding higher productivity could be achieved by controlling the surrounding thermal environment to values of animal thermal comfort.