Up to 30 per cent of pregnant women suffer from leg cramps. The cause of these cramps is not known, but changes in calcium concentration have been suggested. Therefore 42 pregnant women with leg cramps were studied. No differences in total serum or ionized serum calcium concentrations were found as compared with a control group of pregnant women without leg cramps. Twenty-one patients were treated with 1 g calcium orally twice daily for 2 weeks and in this group good clinical improvement was achieved (p less than 0.001). The treatment increased the total serum calcium concentration from 2.25 mmol/l to 2.30 mmol/l but did not alter the ionized serum calcium concentration. Twenty-one untreated patients had the same frequency of cramps and showed no change in serum calcium concentrations throughout the investigation.