Nail sodium, calcium, and magnesium levels were measured in 15 children with cystic fibrosis (CF) of the pancreas and compared with values found in 78 controls. There was a significant increase in the mean value of all three parameters in the CF group. There was an overlap in the values between the affected and control groups for all three measurements. Summing the sodium and calcium values or the sodium, calcium, and magnesium values gave a clear-cut separation between the groups, this being better in the latter case. It is felt that this approach might also prove fruitful in the detection of the heterozygote.