Background: The effects of tailored intervention on multiple behaviors and possible moderators of tailoring effects have not yet been sufficiently demonstrated. Purpose: The purpose of this study was to examine the effectiveness of a computer-tailored intervention on smoking; physical activity; and fruit, vegetable, and fat intake; and to test potential moderators of the effectiveness (BMI, age, SES, gender, motivation, and the number of behaviors for which respondents met the recommendations from national guidelines). Methods: Respondents were randomly assigned to a tailored intervention group, receiving one tailored letter on all of these behaviors, or a control intervention group, receiving one general information letter on all behaviors. Results: Three months after the baseline assessment, the tailored intervention group showed significantly better effects than the control group for all behaviors studied, except for smoking. Notably, the intervention did not enhance the health behaviors, but rather reduced a decline in these behaviors during the 3-month study interval. Effect sizes were small. No moderating factors were found, except for the number of behaviors for which recommendations were met in the tailoring intervention group on fruit consumption. The largest effects of the tailored intervention were found for fruit in respondents who did not meet the recommendations for any behavior (Cohen's d = 0.3). Conclusions: A tailored intervention on multiple behaviors had significant, but limited effects when compared to generic information. The number of bad habits influenced the effects of the tailored intervention on fruit consumption.