During pregnancy, a number of factors predisposing to hyperthermia, such as sleeping in a water bed, sauna bathing, heatwave, high workplace temperature, and fever, are believed capable of harming the foetus. This study aimed to clarify possible associations of maternal workplace temperature, season of birth, sauna bathing, upper respiratory infection, and fever, with cardiovascular malformations in the offspring. The material was composed of all 573 cardiac malformations registered in Finland in 1982-84, and 1055 randomly chosen healthy controls. The mothers were interviewed at maternity welfare centers about three months after delivery. Fever (greater than or equal to 38 degrees C) during early pregnancy was more prevalent among case mothers (10%) than controls (6%) (p less than 0.01), largely manifest as increased risks of atrial septal defect and hypoplastic left heart. Upper respiratory infection was also more common among case mothers (26%) than controls (18%) (p less than 0.001). By contrast, maternal workplace temperature, sauna bathing, and temperature of the environment (seasonal variation) were not associated with risk of cardiac malformation in the offspring. The use of acetylsalicylic acid-containing medication for reducing fever was equally common (7%) among case and control mothers.