AIMS--To look for differences between matched pairs of patients and controls in concentrations of various plasma constituents which might indicate dysfunctions associated with cataract. METHOD--One thousand patients were taken from the cataract waiting list of a specialist eye hospital. For each patient a matched control of the same sex and half-decade of age but without cataract was taken from the patient list of the family doctor of the patient; the control was the next alphabetically after the patient on the doctor's list. The patients and controls were visited in their homes by a team of nurses who performed venepunctures and collected information for a questionnaire. Eye examinations were performed by a team of ophthalmologists. RESULTS--Significant differences were found between the cataract and control groups in 10 of the 18 examined plasma constituents. A constellation of three--bilirubin, alkaline phosphatase, and gamma glutamyl transpeptidase--was significantly higher in the cataract group, suggesting subclinical liver dysfunction as a risk factor. Steroid treatment and diabetes increased cataract risk. Endogenous basal plasma cortisol levels were raised in the cataract group, irrespective of steroid use and diabetic status. Alkaline phosphatase, calcium, glucose, and sodium were all raised in the cataract group. Given the raised total protein and albumin also found in the cataract group, the lower albumin/(total protein-albumin) ratio (an approximation for albumin/globulin ratio) may imply an increase in globulin, suggestive of possible (chronic) infection. Total cholesterol was lower in the cataract group. CONCLUSION--Human cataract in older age groups seems to be due to an accumulation of risk factors, even if individual mean concentrations are well within normal limits but, of course, differing significantly from the corresponding means in the control population.