Nonradioactive in situ hybridization (NISH) on sections of paraffin-embedded neuroblastoma tissue was performed to evaluate numerical and structural aberrations of chromosome 1. Two biotinylated probes specific for the heterochromatic (D1Z1) and subtelomeric regions of chromosome 1 (D1S32) were used to study normal tissue and 4 neuroblastoma samples with and without Ip36 deletions. The NISH findings in 3 of the 4 neuroblastomas correlated well with the results obtained by cytogenetic banding analysis. In 1 tumor sample, however, a deletion at Ip36 was observed by NISH, both on metaphase spreads and interphase nuclei, but not by cytogenetics. The NISH method is therefore advantageous when only paraffin-embedded material is available and can be even more sensitive than conventional cytogenetic analyses under certain conditions. Moreover, the technique provides morphological information that cannot be obtained by methods relying on tissue extracts or cell suspensions.