Between 1976 and 1996, 176 borderline ovarian tumours were registered in the Cancer Registry of the Swiss canton of Vaud, corresponding to an age-adjusted incidence (world standard) of 2.7 in 100,000. Incidence rose from 1.7 per 100,000 during 1976-81 to 2.7 per 100,000 during 1987-91, and then levelled off; 58% of cases were serous and 41% mucinous. Relative survival was 94% at 10 years; 18 second neoplasms were observed, compared with 10.3 expected, and there was a significant excess of invasive ovarian cancers (four observed, including three synchronous, compared with 0.4 expected).