DNA copy number changes were present in most breast cancers with an average of 6.7 aberrations per case (range 0-20). The most frequent changes were losses of 8p, 13q and 16q and gains of 1q, 3q, 8q, 1q and 20q. Amplifications were detected in 25 (32%) cases, most frequently at 11q13, 17q12 and 20q13. Grade 1 well differentiated carcinomas averaged 3.6 aberrations per case and amplifications were detected in 18%. The most frequent changes identified were at 1q (55%), 8q (27%) and 16q (45%). Tubular and tubulo-lobular carcinomas had a similar average number of aberrations, rate of amplifications and losses and gains of 16q and 1q respectively. The grade 2 tumours contained an average of 6.4 aberrations with 25% showing amplifications. The chromosome regions most commonly affected were 1q (65%), 3q (25%), 8p (25%), 8q (21%), 13q (20%) and 16q (55%). The poorly differentiated grade 3 tumours averaged 8.4 aberrations per case, 38% revealing amplifications. The most frequent chromosomal alterations were losses of 8p (46%) and 13q (31%) and gains of 1q (88%), 3q (34%), 8q (73%), 17q (46%) and 20q(31%).FISH analysis on three grade 3 ductal carcinomas with 16q losses demonstrated aneuploidy and in one case amplification of 11q13. Losses at 16q were predominantly found in hormone-positive tumours whereas oestrogen receptor-negative tumours showed gains of 1q,3q,8q,17qand 20q and losses of 13q.