The study included 93 patients with grade 1-3 chronic obstructive pulmonary disease (COPD) in the stable clinical phase. Absorptive function of the small intestine was evaluated by biochemical methods and with the use of radionuclides; plasma tumour necrosis factor-alpha (TNF-alpha) levels were measured by an enzyme immune-assay. Trophologic status in 44% of the patients with COPD was unaltered whereas 14% had an excessive body weight. Trophologic insufficiency (TI) was diagnosed in 42% of the patients. Severe TI or cachexia occurred in 5.4% of the patients and only in severe COPD. Impaired absorptive function of the small intestine documented in moderately-severe and severe phases of COPD correlated with the severity of the disease. There was close relationship between body weight deficit and reduced protein absorption (r = 0.77 +/- 0.09). The presence and progress of TI correlated with the magnitude of increment of plasma TNF-alpha. The level of TNF-alpha in patients with COPD and cachexia was 7.8 times its normal value. Elevated plasma TNF-alpha concentration positively correlated with body weight deficit (r = 0.77 +/- 0.12). Also, significant direct correlation was found between the rise in TNF-alpha concentration in patients with COPD and TI and enhanced excretion of fat measured by the method of Van de Kamer (r = 0.56 +/- 0.12), albumin (r = 0.78 +/- 0.12), and d-xylose (r = 0.46 +/- 0.12). It is concluded that impaired absorption of fats, proteins, and carbohydrates in the small intestine plays an important role in the mechanisms underlying the development of trophologic insufficiency in patients with moderately-severe and severe phases of COPD.