['To look for differences in levels of various plasma constituents between pair-matched controls and patients who had cataracts classified by location and appearance of lens opacity and nuclear colour in order to identify systemic risk factors.', "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. At an early morning visit to the homes of both patients and controls, fasting, a team of nurses performed venepunctures and collected information for a questionnaire. Eye examinations were performed by a team of ophthalmologists.", 'Predominantly nuclear cataract was significantly associated with raised plasma alanine aminotransferase and bilirubin, posterior subcapsular cataract with increased calcium and urea, cuneiform with reduced potassium, mature/hypermature with raised potassium and reduced total carbon dioxide. The following were consistently significantly associated with all forms of cataract; diabetes and raised plasma glucose (not in non-diabetics), use of steroid medication, raised levels of cortisol (steroid users excluded), albumin, alkaline phosphatase, gamma-glutamyl transpeptidase, sodium and total protein and reduced levels of cholesterol and albumin/(total protein-albumin) ratio (an approximation for the albumin/globulin ratio). The multivariate analysis indicated that the most important non-specific cataractogenic effects were those of increased total protein, diabetes and use of steroid medication.'] Do some plasma constituents correlate with human cataract location and nuclear colour?