In a Danish nationwide prospective study of in situ carcinoma of the breast, a total of 275 women, treated with excision alone, were registered from 1982 to 1989. The series included 142 cases of ductal carcinoma in situ (DCIS), 100 cases of lobular carcinoma in situ (LCIS), 26 cases of DCIS+LCIS, and seven cases of atypical hyperplasia (AH). Within a median follow-up of 120 months, a crude recurrence rate of 28% (76 cases) was found, of which 53% (40 cases) recurred as invasive carcinomas (IC) and 47% (36 cases) as CIS. CIS recurrences appeared after median 18 months, compared to median 42 months for IC recurrences. No statistical difference was found with respect to development of IC between the three groups of DCIS, DCIS+LCIS, and LCIS. The majority of recurrences were ipsilateral, also for LCIS. Forty four of 49 recurrences following DCIS, and seven of nine recurrences following DCIS+LCIS occurred as local recurrences. Histopathologically, in DCIS a strong association was found between large nuclear size and comedonecrosis. Univariate analysis showed a significant association to recurrence for nuclear size, comedonecrosis, and size of the original lesion. Multivariate analysis showed that only comedonecrosis and size of lesion were independent predictors of recurrence, however, specimen margins were not included in the analysis, as this parameter could not be adequately evaluated in the present series. Nuclear size of original DCIS lesion was related to histologic grade of the IC recurrence. The recurrence rate for DCIS of small nuclear size increased from 6% at five years of follow-up to 16% at 10 years, possibly due to a slower growth rate and a continued but delayed risk. Similarities were found between LCIS and DCIS of small nuclear size, both showing a continued risk and comparable rate of recurrence. Further, progression to IC of similar, highly differentiated type was seen, indicating a linkage between biological behavior of the two histological types.