Middle-aged and elderly females aged between 51 and 86 years volunteered for this study. In June and July, 147 females were studied; data collected through interview included general socioeconomic information, health history, use of medication, and a 7-day food intake. During the same 7-day period, physical activity level was measured for each person by use of a pedometer. On the 8th day, anthropometric measurements were conducted and fasting blood was analyzed for certain hormones and related metabolites. In the following January and February, the same group was surveyed again and the same measurements were repeated. Of the initial 147 subjects, 130 persons completed the winter survey. A comparison of the summer and winter data indicated some significant differences: mean body weight increased in winter from 70.4 +/−12.7 to 71.1 +/−13.5 kg (p less than or equal to 0.05); energy intake was raised from 1450 +/−384 to 1549 +/−372 kcal (p less than or equal to 0.01); physical activity decreased from 10.9 +/−8.2 to 8.5 +/−5.4 miles/week (p less than or equal to 0.01); and serum free thyroxine (T4) fell from 1.72 +/−0.27 to 1.64 +/−0.27 ng/dl. These results together with other data suggest that more attention should be addressed to seasonal fluctuations involved in body weight maintenance.