A brief review is given of attempts to interpret the low-angle meridional X-ray diffraction patterns of tendon to yield the axially projected electron density of collagen fibrils. Both X-ray/electron microscope correlations and more conventional crystallographic methods have been used. For wet tendon, there have been two fundamental assumptions; either that those parts of the fibril which are positively stained in electron micrographs are intrinsically electron dense, or that the electron density is centrosymmetric. The validity of these assumptions is tested by synthesising models for the electron density based on the known amino acid sequence of collagen and the Hodge-Petruska scheme. Allowance is made for the effect of the water environment. These models are then Fourier transformed to give a set of calculated intensities for comparison with the observed low-angle meridional X-ray data. The calculated intensities are very sensitive to the conformation of the non-triple-helical terminal peptides (telopeptides) and to the D period. Difference Fourier syntheses are used to refine a model which gives the best agreement with the observed data whilst being both physically and chemically acceptable. The calculated phases of this model (model E) are then combined with the observed amplitudes to give a Fourier synthesis of the axially projected electron density in wet collagen fibrils. The implications of this result for the study of various metabolic and pathological situations in connective tissue are discussed.