This prospective study was carried out in order to evaluate the influence on liver morphology and function of a very-low-calorie formula diet. Fourty-one morbidly obese, non-alcoholic subjects had liver biopsy performed before and after a median weight loss of 34 kg. Fatty change improved (p less than 0.001), but 24% of the patients developed slight portal inflammation (p = 0.039) or slight portal fibrosis (p = 0.063). Patients developing portal fibrosis had a higher degree of fatty change at entry (p = 0.029), a more pronounced reduction of fatty change (p = 0.014) and a faster weight loss (p = 0.026). Liver biochemistry, which was of no individual diagnostic value, improved. It is concluded that morbidly obese subjects with a high degree of hepatic fatty change are at risk of developing portal inflammation and fibrosis when undergoing very fast dietary weight reductions.