OBJECTIVE--To determine the sociodemographic and motivational characteristics of parents who volunteer their children for clinical research. DESIGN--A questionnaire was administered to parents who volunteered their children for a randomised, double blind, placebo controlled trial of a drug to treat asthma and to a control group of parents whose children were eligible for the trial but had refused the invitation. SETTING--A children's hospital in Australia. SUBJECTS--68 Parents who had volunteered their children and 42 who had not; a response rate of 94% and 70%, respectively. MAIN OUTCOME MEASURES--Responses of parents to questionnaire designed to assess their perceptions, attitudes, and health seeking behaviour as well as sociodemographic data. RESULTS--Volunteering parents were less well educated with only 15% (10/68) of mothers and 16% (11/68) and of fathers having had a tertiary or university education compared with 26% (11/42) of mothers and 45% (19/42) in the non-volunteering group. Fewer volunteering parents had professional or administrative jobs than did non-volunteering parents (mothers 6% (4/68); fathers 9% (6/68) v mothers 14% (6/42); fathers 31% (13/42)). Volunteering parents had less social support, and they displayed greater health seeking behaviour and consumed more habit forming substances. They were motivated by a desire to help others and to contribute to medical research, but they were also searching for more information and better ways to help their own children. CONCLUSION--Parents who volunteer their children for medical research are significantly more socially disadvantaged and emotionally vulnerable.