1,25 dihydroxyvitamin D3, the active form of vitamin D, has immunomodulatory properties in vitro and in vivo. We report that treatment with 1,25 dihydroxyvitamin D3 (5 micrograms/kg on alternate days) prevents the development of clinical diabetes in NOD mice, an animal model of human autoimmune diabetes. Diabetes incidence in female NOD mice at the age of 200 days was reduced to 8% in the 1,25 dihydroxyvitamin D treated group vs 56% in the control group (p < 0.0001). In parallel, treatment with 1,25 dihydroxy-vitamin D3 resulted in a complete normalisation of the capacity to induce suppressor mechanisms in an autologous MLR, which is severely depressed in control NOD mice. The existence of such suppressor cells was confirmed in transfer experiments, whereby cotransfer of splenocytes from 1,25 dihydroxyvitamin D3 treated NOD mice prevented diabetes transfer by splenocytes from diabetic NOD mice into irradiated, 6-8-week-old male NOD mice. Other known immune defects of the NOD mice, such as defective natural killer cell killing of YAC-1 targets and defective thymocyte activation by anti-CD3 were not corrected. The pharmacological doses of 1,25 dihydroxyvitamin D3 were universally well tolerated as reflected by a normal weight gain of the mice. Serum calcium was increased (2.5 +/- 0.2 vs 2.2 +/- 0.2 mmol/l in the control group, p < 0.005), whereas osteocalcin levels nearly doubled and bone calcium content was halved. These findings show that 1,25 dihydroxyvitamin D3 can prevent diabetes in NOD mice, probably through the correction of their defective suppressor function.