The dietary intake of micronutrients and serum micronutrient status have been topics of concern in relation to human immunodeficiency virus (HIV) progression. Most data, however, were collected prior to the introduction of protease inhibitors (PIs). We analyzed dietary intake and serum values of vitamin B(12), including the effect of PIs, in a cohort of persons with HIV infection. During intervals with no PI use, each 1 microg/day increase in B(12) intake was associated with a 1.06 pg/mL increase in serum B(12) levels. However, during intervals with PI use, each 1 microg/day increase in intake was associated with only a 0.12 increase in serum B(12) levels. Adequate serum B(12) levels (>350 pg/mL) cannot be assumed even in the presence of PIs, and dietary supplementation may not be adequate to significantly increase serum B(12) levels. Serum B(12) levels should be determined yearly in persons with HIV infection, regardless of whether they are receiving PI treatment.