Mechanisms of selective attention exert a powerful influence on visual perception. We examined whether attentional selection is necessary for generation of the vivid colours experienced by individuals with grapheme-colour synaesthesia. Twelve synaesthetes and matched controls viewed rapid serial displays of nonsense characters within which were embedded an oriented grating (T1) and a letter-prime (T2), forming a modified attentional blink (AB) task. At the end of the stream a coloured probe appeared that was either congruent or incongruent with the synaesthetic colour elicited by the letter-prime. When the prime was attended, synaesthetes showed a reliable effect of prime-probe congruency. In contrast, when the prime appeared at 350 ms following T1 (during the AB), the congruency effect was eliminated. Our findings suggest that focused attention is crucial for inducing letters to elicit colours in synaesthesia.