Slivers of hoof horn were taken from the outer claw of the right hindfoot of 16 first calving dairy heifers one month before calving and one, 10 and 20 weeks after calving. The gross appearance of all four feet, and the presence and degree of solar haemorrhage were recorded. Ultra-thin sections of the white line were examined in the transmission electron microscope. Good quality horn showed good cellular structure and no amorphous or filamentous intercellular material. Horn of moderate quality showed areas of material in the intercellular spaces which caused the individual cells to separate and were accompanied by a loss of internal, cellular structure. Poor quality horn showed large areas of cellular destruction, red blood cells in intercellular spaces and a disruption of the pattern of keratinisation. The ultrastructural appearance of the white line was a sensitive indicator of the quality of the horn. At the start of the study all the heifers had good feet with minimal or no solar haemorrhage. Four of the eight heifers with poor quality horn one month before calving went on to develop severe solar haemorrhage 10 to 20 weeks after calving, two developed moderate (grade 3) haemorrhage and two developed slight haemorrhage. The eight heifers with good quality horn at the start of the study all showed little or only moderate solar haemorrhage by 20 weeks after calving.