The impact of maturation factors and functioning level on the relationship between disabled and non-disabled siblings was examined. Two groups of adults, distinguished by level of functioning of their mentally retarded sibling, completed a Schaefer Sibling Behavior Inventory (SBI) and participated in a semi-structured interview. The latter explored a number of dimensions of the relationship including degree of warmth, contact and involvement. Judgements about involvement and comfort as remembered from the past and judgements about the present were obtained to derive hypotheses about possible changes with time. Data from the SBI indicated that significantly more competent siblings tended to be involved in relationships with a higher degree of reciprocity. In contrast, measures of attitude derived from interview data did not appear to relate to the functioning level of the sibling in any systematic manner. The results suggest that level of discomfort with peers may decrease significantly over the years with neither level of functioning nor gender being important variables. The extent to which non-disabled individuals expressed a preparedness to participate in the life of their disabled sibling may relate to the life-stage of the two. Males in particular anticipated increased future involvement. Although higher-functioning siblings were described as more active in the relationship, this did not appear to influence perceived positiveness of regard. The implications of these results are discussed briefly.