This study examined the early phase transplant coronary artery disease (TxCAD), as measured by the quantitative coronary angiography (QCA) mean lumen diameter loss (MLDL). A comparison was made between the large primary versus the smaller branch vessels. The relationship of different non-immunological and immunological factors to the development of TxCAD was examined, and we correlated the QCA data to that obtained from intracoronary ultrasound (ICUS). 121 patients who were transplanted between September 1994 and June 1999 were studied and followed up for a period of one to five years. 103 patients were males (85%) and the mean age was 48.5 10 years. We found that TxCAD was predominantly a disease of the large vessels, as evidenced by greater MLDL in the first years after transplant. The MLDL increased with time in both the large and small vessels and the greatest loss occurred in the first year. The first year MLDL was a predictor of long-term MLDL. The relative changes in lumen diameter measured by QCA and ICUS were similar. A low early left ventricular echocardiographic ejection fraction was related to greater MLDL in the large vessels as was donor male sex. Domino and non-domino hearts did not differ in the long-term MLDL. Total ischaemic time, RATG induction of immunosuppression, baseline vessel tone, acute rejection, CMV infection, total number of HLA mismatches and the first year mean antivimentin antibodies level, were not related to MLDL, nor was the baseline right and left heart catheter haemodynamic data. In conclusion, TxCAD as measured by QCA-MLDL was predominantly a disease of the large epicardial vessels and was predicted by a low early echocardiographic ejection fraction. Both QCA and ICUS could be used to assess TxCAD and their measurements were well correlated.