OBJECTIVE To investigate if water-gymnastics during pregnancy may reduce the intensity of back/low back pain and the number of days on sick-leave. METHODS A prospective, randomized study. One hundred and twenty-nine women were randomized to participate in water-gymnastics once a week during the second half of pregnancy and 129 were randomized to a control group. The women in both groups filled in questionnaires in gestational weeks 18, 34 and within the first postpartum week. Every day from week 18 to labor they assessed the intensity of back/low back pain. RESULTS Back pain intensity increased during pregnancy. No excess risk for the pregnancy associated with water-gymnastics was observed. The women participating in water-gymnastics recorded a lower intensity of back/low back pain. The total number of days on sick-leave because of back/low back pain was 982 in the water-gymnastics group (124 women) compared with 1484 in the control group (120 women). After weeks 32 33, seven women in the water-gymnastics group compared with 17 in the control group were on sickleave because of back/ low back pain (p=0.031). CONCLUSIONS Intensity of back/low back pain increased with advancing pregnancy. There was no excess risk for urinary or vaginal infections associated with water-gymnastics. Water-gymnastics during the second half of pregnancy significantly reduced the intensity of back/ low back pain. Water-gymnastics decreased the number of women on sick-leave because of back/low back pain. Water-gymnastics during pregnancy can be recommended as a method to relieve back pain and may reduce the need for sick-leave.