Ataxia was diagnosed in kids from a New England goat herd. Concurrent infection with the caprine arthritis/encephalitis (CAE) virus contributed to the development of hind limb ataxia and weakness in one of the kids. Six kids from this herd had signs of hind limb ataxia and paralysis. Detailed evaluation of 2 of the affected kids revealed low liver and serum copper concentrations and spinal cord demyelination. One kid also had histologic changes in the CNS and lungs, compatible with a diagnosis of CAE. Serum copper concentration was determined in affected goat kids and their dams and was compared with serum copper concentration in clinically normal kids and their dams from the same herd. Serum copper concentration also was measured in dams and kids in a control herd that had no history of ataxia. The mean serum copper concentration in affected kids was 0.125 microgram/ml, compared with 0.45 microgram/ml in unaffected kid herdmates. Kids from the control herd had mean serum copper concentration of 0.6 microgram/ml. Mean serum copper concentration in dams of kids with neurologic signs also was low (0.25 microgram/ml), compared with that (0.5 microgram/ml) in dams of clinically normal kids of the affected herd and that (0.95 microgram/ml) in dams of kids of the control herd. Results of a serologic survey (by use of agar gel immunodiffusion) of the affected herd for CAE indicated that 69.5% of the goats were seropositive. Dietary copper intake was determined to be adequate in this goat herd; therefore, copper deficiency appeared to be conditioned by an interfering substance. However, a search for interfering substances was unrewarding.