An immunocytochemical assay (ICA) for the measurement of estrogen receptor (ER) and progesterone receptor (PgR) has been evaluated in 426 human primary breast carcinomas. For estrogen receptor determination ER ICA was used. PgR ICA was performed using the monoclonal antibody KD 68. Assay results for progesterone receptor immunocytochemistry were in agreement (P < 0.0001) with those of biochemical determination in 74%. Progesterone receptor positivity determined with a semiquantified approach based on intensity and heterogeneity of immunocytochemical staining correlated significantly with biochemically determined progesterone receptor levels (P = 0.0001). Survival data showed a significantly better overall survival for patients with either ER ICA- or PgR ICA-positive carcinomas (ER ICA, P < 0.00001; PgR ICA, P = 0.004). Patients with both negative ER ICA and PgR ICA showed a poorer prognosis than patients with only one negative receptor. In ER ICA- and PgR ICA-positive carcinomas a trend could be found that patients whose carcinomas contained high numbers of receptor-positive tumor cells had a better survival. This study demonstrates that ER ICA and PgR ICA are strong prognostic indicators and that the proportion of steroid hormone receptor-positive tumor cells seems to be of clinical importance.