Idiopathic developmental disability (DD) has been found to put significant psychological distress on families of children with DD. The cause of the disability, however, is unknown for up to one-half of the affected children. Chromosomal abnormalities identified by cytogenetic analysis are the most frequently recognized cause of DD, although they account for less than 10% of cases. Array genomic hybridization (AGH) is a new diagnostic tool that provides a much higher detection rate for chromosomal imbalance than conventional cytogenetic analysis. This increase in diagnostic capability comes at greater monetary costs, which provides an impetus for understanding how individuals value genetic testing for DD. This study estimated the willingness to pay (WTP) for diagnostic testing to find a genetic cause of DD from families of children with DD. A discrete choice experiment was used to obtain WTP values. When it was assumed that AGH resulted in twice as many diagnoses and a 1-week reduction in waiting time compared with conventional cytogenetic analysis, this study found that families were willing to pay up to CDN$1118 (95% confidence interval, $498–1788) for the expected benefit. These results support the conclusion that the introduction of AGH into the Canadian health care system may increase the perceived welfare of society, but future studies should examine the cost-benefit of AGH vs cytogenetic testing.