OBJECTIVES: To examine the effects of survey mode of administration on data quality when using a mixed-mode (i.e., paper-mode vs. web-mode) approach.  METHODS: A cross-sectional wave of 4,836 participants active in the Cancer of the Prostate Strategic Urologic Research Endeavor (CaPSURE) registry, self-selected to complete the Medical Outcomes Short-Form 36 instrument.  Data quality parameters compared response rate; data completeness at the item and computable scale score; differences in mean scale scores; and range of values (i.e., ceiling and floor effects) by mode.   RESULTS: The majority of participants, 4,376 (90%), opted for paper-mode.  Overall response rate was 77%; paper-mode 76% versus 88% web-mode (p <.01).  Paper-mode respondents were older, 70.7 (SD 8.8) versus 66.8 (SD 8.5) (p <.001).  Paper-mode participants had significantly more missing items (mean .67 vs. .10, p <.0001) and significantly lower proportion of computable scale scores for bodily pain, general health, vitality, social functioning, and mental health (p < .03).  The social functioning scale had the largest difference for the proportion with missing data, 92.3% vs. 99.5% (p = .00).  Mean scale scores were lower among paper-mode participants for all eight scales and the physical and mental component summary scores (p <.03).  Significantly lower mean scale scores with small differences in effect size were noted for paper versus web-mode for the physical component summary, 47.5 vs. 50.2 (D = .30); physical functioning, 80.2 vs. 88.2 (D = .36); and vitality, 64.8 vs. 71.2 (D = .32).  Four scales demonstrated a full range of values (0 to 100).  For the web-mode, four scales did not achieve a floor value:  Vitality (10 – 100); mental health (28 – 100); bodily pain 23 – 100; and general health (15-100).  CONCLUSIONS: We observed significant differences in data quality with mixed-mode survey administration but effect-size differences were small suggesting that mixed-mode administration did not introduce significant measurement differences.