The tolerability of supplemental iron in the chelated form of bis-glycino iron II was compared with that of ferrous sulfate in a randomized, double-blind, cross-over trial. Both iron formulations were prepared to deliver 50 mg elemental iron in each capsule; the capsules for both formulations were identical in appearance and weight. Each supplement was taken once daily before breakfast for two weeks. The incidence and severity of adverse side effects were not statistically different for the two preparations. However, of the 38 women evaluated, 14 (37%) experienced moderate-to-severe side effects only while taking the sulfate formulation compared to eight (21%) who experienced similar side effects only while taking the chelate formulation; the remaining 16 women had the same symptom profile with both preparations. This tendency for the chelate to be better tolerated was observed for the symptoms of bloating, constipation, and nausea. In addition, a significant (P less than 0.05) number of women preferred the chelate to the sulfate formulation. This preference appeared to be related to the lower number of side effects experienced with the chelate.