This study assessed age and sex effects on muscle fibre adaptations to heavy-resistance strength training (ST). Twenty-two young men and women (20–30 years old) and 18 older men and women (65–75 years old) completed 9 weeks of heavy-resistance knee extension exercises with the dominant leg 3 days week−1; the non-dominant leg served as a within-subject, untrained control. Bilateral vastus lateralis muscle biopsies were obtained before and after ST for analysis of type I, IIa and IIx muscle fibre cross-sectional area (CSA) and fibre type distribution. One-repetition maximum (1-RM) strength was also assessed before and after ST. ST resulted in increased CSA of type I, IIa and IIx muscle fibres in the trained leg of young men, type I and IIa fibres in young women, type IIa fibres in older men, and type IIx fibres in older women (all P < 0.05). Analysis of fibre type distribution revealed a significant increase in the percentage of type I fibres (P < 0.05) along with a decrease in type IIx fibres (P= 0.054) after ST only in young women. There were no significant changes in muscle fibre CSA or fibre type distribution in the untrained leg for any group. All groups displayed significant increases in 1-RM (27–39%; all P < 0.01). In summary, ST led to significant increases in 1-RM and type II fibre CSA in all groups; however, age and sex influence specific muscle fibre subtype responses to ST.