The concentrations of anticardiolipin immunoglobulin G (IgG) were measured in 134 healthy dogs and 63 diseased dogs by an ELISA. The mean (sd) concentration in the healthy dogs was 5·40 (2·60) IgG phospholipid (GPL) units, and concentrations greater than 11 GPL were considered as above the normal range. On this basis, 30 (47·6 per cent) of the diseased dogs were within the normal range, with a mean of 5·45 (3·07) GPL and the other 33 had levels above the normal range (P<0·001); 19 of them had a mean level of 22·2 (5·66) GPL, 10 had a mean level of 49·1 (11·2) GPL, and four had a mean level of 85·8 (9·64) GPL. Levels above the normal range were more frequent in females (59·4 per cent) than in males (45·1 per cent), but were higher in males (45·5 [34·71] v 42·91 [22·0] GPL). In addition, they were more frequent and higher in older dogs (66·7 per cent, 40·4 [24·0] GPL) than in younger dogs (33·3 per cent, 33·5 [21·4] GPL).