BACKGROUND: Most blood donor deferrals are temporary and short-term. The effect of short-term, temporary deferral (STTD) on blood donor return rates and subsequent blood donations is an important issue. STUDY DESIGN AND METHODS: Donors given STTDs during the first 3 months of 1993 were computer-matched with nondeferred donors on the basis of age, sex, and donation date. Computer records were evaluated during the next 4.25 years (4/93-6/97) to determine donor return rates and subsequent blood donations. RESULTS: The most common reasons STTD were low hemoglobin (46%), colds and/or sore throats (19%), and elevated temperature (10%). Nondeferred donors were 29 percent more likely than donors with STTD to return over the next 4.25 years (80% vs. 62%), and nondeferred donors donated 81 percent more whole blood units (13,798 vs. 7,615) over the same period. CONCLUSION: The study showed that STTD have a very negative impact on blood donor return rates and subsequent blood donations. Actions to alleviate these negative effects are indicated.