Angiographic morphology was analysed in 32 patients who developed restenosis after initially successful coronary angioplasty. The mean minimal luminal diameter of the dilated coronary segments increased from 0.9 mm to 2.3 mm after dilatation, but decreased to 0.9 mm at restenosis. The reference diameter was unchanged after dilatation and at restenosis. Mean stenosis length before the initial angioplasty was 7.0 mm but at the repeat procedure had increased to 8.7 mm (mean increase 1.7 mm, 95% confidence interval 0.6 to 2.8 mm, p less than 0.01). There were no significant differences in mean trans-stenotic pressure gradient and mean eccentricity ratio between the initial and repeat angioplasty procedures. In individual patients the changes in stenosis morphology were unpredictable, but overall stenoses tended to be longer at restenosis. In some patients stenosis length increased by several millimetres but the success rate of repeat angioplasty was high and the clinical importance of the changes in stenosis morphology are uncertain.