In 34 patients with acute myocardial infarction (MI) plasma pyridoxal-5'-phosphate (PLP) levels were significantly lower (5.22 +/- 1.88 ng/ml) than those in an age- and sex-matched control group (11.5 +/- 2.03 ng/ml). In another group of patients who had clinical and angiographic evidence of ischaemic heart disease but had not had an MI plasma PLP levels were not significantly different from those in the control group (10.07 +/- 2.98 ng/ml). However, plasma high-density lipoprotein cholesterol levels in this group (0.75 +/- 0.28 mmol/l) as well as in the MI group (0.76 +/- 0.28 mmol/l) were significantly lower than those in the control group (1.26 +/- 0.23 mmol/l). On follow-up, all of 15 patients who had had an acute MI showed a continuous decrease in plasma PLP levels of approximately 50% during the first 48 hours after admission. Sixteen healthy volunteers subjected to a period of prolonged fasting (+/- 30 hours) displayed a decrease of approximately 43% over this period. We conclude that an acute reduction in plasma PLP levels occurred during the acute phase of MI. Other factors, for example prolonged acute starvation, may also produce a rapid decrease in plasma PLP levels.