To examine whether dietary supplementation with omega-3 fatty acids from Clupeonella grimmi can relieve symptoms of dysmenorrhoea, we carried out a cross-over clinical trial on 36 girls aged 18-22 years. They were randomly allocated into 2 groups of 18. Group A received 15 mL fish oil daily (550 mg eicosapentaenoic acid; 205 mg decosahexaenoic acid) while Group B received placebo. After 3 months, the treatment regimens were swapped. The treatment groups reported a significant difference after 3 months of supplementation with fish oil (visual analogue scale score 20.9 compared with 61.8 for the placebo (P= 0.001). There was also a marked reduction in low back pain and abdominal pain (P < 0.05), and participants needed significantly fewer rescue doses of ibuprofen while using fish oil.