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Objective To examine the absolute risks or benefits on cancer associated with oral contraception, using incident data.

Design Inception cohort study.

Setting Royal College of General Practitioners9 oral contraception study.

Participants Directly standardised data from the Royal College of General Practitioners9 oral contraception study.

Main outcome measures Adjusted relative risks between never and ever users of oral contraceptives for different types of cancer, main gynaecological cancers combined, and any cancer. Standardisation variables were age, smoking, parity, social class, and (for the general practitioner observation dataset) hormone replacement therapy. Subgroup analyses examined whether the relative risks changed with user characteristics, duration of oral contraception usage, and time since last use of oral contraception.

Results The main dataset contained about 339 000 woman years of …