During prolonged starvation blood gas analyses on 24 lean and 78 obese subjects were performed. A statistically significant sex difference of acid-base status was found in that lean females were more acidotic after 4 - 6 days of fasting than lean males. This sex difference was abolished by severe and long-standing obesity, since females suffering from this condition were not significantly different from lean and obese males. Out of 78 obese patients fasted for 1 - 4 weeks 8 had one or more pH values below 7.30. It is concluded that regular blood gas analyses are indicated in all patients subjected to prolonged total fasting.