Selective subject attrition from longitudinal study panels can bias estimates of developmental change. Particularly in studies of older adults, sampling effects can adversely affect attempts to estimate true ontogenetic change. Selective attrition effects were examined in 636 Pennsylvania adults (138 males, 498 females), aged 58-91, who were tested in 1978-1979; and 232 subjects who returned and were retested in 1986-1987. On both occasions, subjects received measures of intellectual ability, locus of control beliefs, and attitudes toward aging. Comparison of the Time One ability performances of returning and non-returning subjects indicated significantly lower performance levels for dropouts on measures of Verbal Ability, Figural Relations, Induction, Experiential Evaluation, Memory Span, and Perceptual Speed (p<.05). Logistic multiple regression procedures identified significant control belief and demographic predictors of attrition status ("R"=.25) and attrition type ("R"=.327). The group of non-returning subjects tended to be older, contained a higher proportion of males, were more likely to believe that chance controlled intellectual performance, had lower intellectual achievement motivation levels, were less likely to be employed, and reported lower levels of subjective health. The effects of these predictors were partialled out of the abilities showing significant attrition effects; this eliminated the significant attrition effect on four of the six abilities. These results suggest a possible procedure for examining selective attrition effects and quantifying sample bias in longitudinal studies of the elderly. (Author/NB)