To assess whether serum creatinine and age are associated with headache induced by nitrates, 2742 hospitalized patients taking nitrates were studied during their hospital stay. Those patients with admission serum creatinine levels from 97 to 133 mumol/L and > 133 mumol/L were compared with patients with creatinine levels < 97 mumol/L. Gender, body mass index, comorbidity, cognitive status, new intake of nitrates, number of daily administrations, and daily dosage, as well as intake of angiotensin converting enzyme inhibitors, calcium antagonists, diuretics and nonsteroidal anti-inflammatory drugs were examined as possible confounders. Fifty-six patients had headaches that had a causal link with intake of nitrates. Compared with the lowest creatinine group, after adjustment for potential confounding variables, the odds ratios and 95% confidence interval (95% CI) for headache caused by nitrates associated with increasing serum creatinine levels were 0.6 (95% CI, 0.3 to 1.1) and 0.2 (95% CI, 0.0 to 1.2), respectively (p for trend = 0.013). Increasing age was inversely associated with headache (odds ratio for 10-year increase, 0.6 [95% CI, 0.5 to 0.7]). Serum creatinine and age were independently and inversely associated with headache caused by nitrates.