Background: Developmental reading problems show strong persistence across the school years; less is known about poor readers' later progress in literacy skills. Method: Poor (n = 42) and normally developing readers (n = 86) tested in adolescence (ages 14/15 years) in the Isle of Wight epidemiological studies were re-contacted at mid-life (ages 44/45 years). Participants completed a spelling test, and reported on educational qualifications, perceived adult spelling competence, and problems in day-to-day literacy tasks. Results: Individual differences in spelling were highly persistent across this 30-year follow-up, with correlations between spelling at ages 14 and 44 years of r = 0.91 (p less than 0.001) for poor readers and r = 0.89 (p less than 0.001) for normally developing readers. Poor readers' spelling remained markedly impaired at mid-life, with some evidence that they had fallen further behind over the follow-up period. Taking account of adolescent spelling levels, continued exposure to reading and literacy demands in adolescence and early adulthood was independently predictive of adult spelling in both samples; family social background added further to prediction among normally developing readers only. Conclusions: By adolescence, individual differences in spelling and its related sub-skills are highly stable. Encouraging young people with reading disabilities to maintain their exposure to reading and writing may be advantageous in the longer term.