The clinical, radiographic and physiological features, and progression rates of forty unselected patients with cryptogenic fibrosing alveolitis (CFA) have been studied in relation to serum immune complexes measured by a C1q binding technique. Twenty (50%) had levels greater than normal. Those with associated connective tissue disorders (twenty-four) had a higher frequency of raised C1q binding than those with 'lone' CFA (sixteen) (63% compared to 31%). As observed previously in this group, those with associated disorders also had higher titres of immune complexes. Twelve of thirteen with polyarthritis had immune complexes and a considerably shorter duration of disease compared with those without joint symptoms (P less than 0.01). Several other observations suggest that immune complexes are especially associated with earlier disease. These include a trend towards a younger age and a lesser radiographic profusion score; a shorter duration of symptoms in relation to titres of immune complexes amongst those with raised values (P less than 0.05) and a higher transfer factor coefficient (Kco) (P less than 0.02). The relationship between Kco and the presence of immune complexes was still observed when those with 'lone' CFA were analysed separately. Other clinical features including sex, severity of dyspnoea and lung volumes did not distinguish those with and without complexes, either for the whole group of patients or when those with and without associated connective tissue disorders were analysed separately. Analysis of eighteen patients followed to death showed no correlation between length of survival from first symptoms and immune complexes, neither was there any clear association with corticosteroid responsiveness and immune complexes.(ABSTRACT TRUNCATED AT 250 WORDS)