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One of the principal peptide components of the amyloid plaque deposits of Alzheimer's disease in humans is the 40‐amino‐acid peptide β‐amyloid A4‐(1–40)‐peptide. The full‐length A4‐(1–40)‐peptide was chemically synthesized and the solution structure determined by two‐dimensional nuclear magnetic resonance spectroscopy and restrained molecular‐dynamics calculations. Synthetic human A4‐(1–40)‐peptide was soluble and non‐aggregating for several days in 40% (by vol.) trifluoroethanol/water. All spin systems could be unambiguously assigned, and a total of 203 sequential and medium‐range cross‐peaks were found in the NOESY (nuclear Overhauser enhancement spectroscopy) spectrum. Long‐range NOE cross‐peaks that would indicate tertiary structure of the peptide were absent. The main secondary‐structure elements found by chemical‐shift analysis, sequential and medium‐range NOESY …