The retest reliability and validity of self-reported gambling behavior were assessed in 2 samples of problem gamblers. Days gambled and money spent gambling over a 6-month timeframe were reliable over a 2- to 3-week retest period using the timeline follow-back interview procedure (N=35; intraclass correlation coefficients [ICCs] ranged from .61 to .98). Gamblers did, however, report significantly more gambling at the 2nd interview. Agreement with collaterals was fair to good overall (ICCs ranged from .46 to .65) with no clear pattern of either over- or underreporting by gamblers. Spouses did not show greater agreement with gamblers compared with nonspouses, and greater agreement was not found for collaterals who were more versus less confident in their reports. The results are generally supportive of the use of self-reported gambling in studies of problem gamblers, assessed face to face and by telephone, although suggestions for further research are provided.