This research focused on the validity of young adults' (mean age=33 years; standard deviation, 3.9) self-reports of reasons for hospitalization and factors affecting validity in a longitudinal cohort study of over 5,000 young adults in four US cities (1985-1998). Self-reported reasons were considered discordant if they differed from those in medical records. Of the 321 self-reported hospitalizations, overall concordance was 92.5%; concordance ranged from 80% for infections to 100% for injuries/fractures and procedures/surgeries. There were no significant differences among mail, telephone, or face-to-face methods of collecting self-reports. In generalized estimating equations analyses, Black race (odds ratio=4.23, 95% confidence interval: 1.72, 10.40; p=0.002) and intravenous drug use (odds ratio=6.06, 95% confidence interval: 1.17, 31.22; p=0.03) were positively associated with discordance. Nonetheless, self-reports by Blacks were 90.0% concordant. Self-reports by Whites were 95.7% concordant. These results suggest that young adults' self-reported reasons for hospitalization are overwhelmingly concordant with medical records. This has important implications, since obtaining medical records has become more costly and logistically difficult.