BACKGROUND Incomplete participation is of particular concern for surveys of patient perceptions of care because patients who have negative opinions may be least likely to participate. OBJECTIVE We sought to examine indirect evidence of nonresponse bias. DESIGN We re-analyzed data from a cross-sectional patient survey. SUBJECTS Our subjects were patients discharged from a Swiss hospital (n = 2156). MEASURES We measured the following: (1) an observed problem score, based on 15 key items of the Picker Patient Experience questionnaire, (2) a predicted problem score, and (3) a participation propensity score. The latter scores were computed for all eligible patients, including those who did not return the survey, from routinely available baseline data. RESULTS The participation rate was 70% (n = 1518), and the mean problem score was 29.9 (SD 23.8). Early respondents reported significantly fewer problems than late respondents (28.6 versus 32.9, P = 0.001). Participation propensity scores were progressively lower in early respondents (mean 74.2), late respondents (70.7), and nonrespondents (63.9, P < 0.001); the pattern was similar for predicted problem scores (early respondents: 29.5; late respondents: 30.5; nonrespondents: 33.4, P < 0.001). The propensity to participate was negatively associated with the problem score (Pearson r = -0.19). Finally, predictors of participation were similar to predictors of problem scores. CONCLUSIONS The tendency to participate in the survey was negatively associated with the report of problems during hospitalization. Nevertheless, increasing participation from 30% to 70% had only a modest influence on the final conclusions of the survey.