The 'two-hit' hypothesis for the development of the childhood eye cancer, retinoblastoma (Rb), predicts that bilaterally affected individuals will carry germline mutations. The second suggestion is that patients with early presentation of unilateral tumours also carry predisposing mutations. We have used SSCP analysis to study the 27 individual exons of the RB1 gene in constitutional DNA from 3 patients whose tumours were treated under the age of 12 months. Bandshifts on SSCP gels were detected in 2 of these patients which, on sequencing, were shown to be a C-->T transition converting a CGAarg to a TGAstop codon in exon 17 and an 8 bp deletion in exon 20 resulting in a downstream stop codon. The mutations seen in these patients are reminiscent of those seen in patients with hereditary Rb and confirms that at least some early onset unilateral cases carry constitutional mutations, which has important implications for genetic screening and counselling of these individuals.