['Vitamin D deficiency has been recently associated with the metabolic syndrome. However, it is not known whether this possible association of vitamin D deficiency with the metabolic syndrome is still present at very high degrees of obesity, as in morbidly obese patients.', 'Transversal, observational study that included 73 consecutive morbidly obese patients (body mass index 40 kg/m(2)). In every patient, anthropometric variables were recorded, fasting blood was assayed for 25-hydroxyvitamin D concentrations, lipid profiles, glucose and insulin levels, and insulin resistance was estimated by homeostasis model assessment.', 'Vitamin D deficiency was present in 37 of the 73 patients (50.7%). As defined by revised Adult Treatment Panel III criteria, 46 of the 73 obese patients (63%) had the metabolic syndrome. Vitamin D deficiency was more prevalent in morbidly obese patients presenting with the metabolic syndrome, compared with those who did not achieve the criteria for this syndrome (60.9% vs. 33.3% respectively, P = 0.023). When serum concentrations of 25-hydroxyvitamin D were categorized in tertiles, there was an association of the prevalence of the metabolic syndrome with the former (P = 0.038). Serum high-density lipoprotein cholesterol concentrations were lower (37.0+/-7.8 mg/dl vs. 44.9+/-8.7 mg/dl, P = 0.003), and triglycerides levels were higher (163.3+/-81.5 mg/dl vs. 95.1+/-24.2 mg/dl, P = 0.001) in the vitamin D-deficient group.'] Is vitamin D deficiency associated with the metabolic syndrome in morbid obesity?