Solid tumors are composed not only of tumor cells but also of stromal nonneoplastic cells. In whole tumor samples, stromal cells retaining their alleles may therefore obscure detection of loss of heterozygosity (LOH) in tumor cells. An increasing number of studies have used laser-assisted tissue microdissection to improve LOH detection, but the real gain in sensitivity has been poorly quantified. We studied a group of 16 inflammatory breast carcinomas that were submitted to both standard DNA extraction from frozen whole tumor samples and laser microdissection performed on paraffin-embedded tumor samples. Using PCR with fluorescence-labeled primers, we comparatively analyzed ten polymorphic markers with both sources of DNA. With the LOH detection threshold set at -25%, we showed that 25 LOHs could not be diagnosed with whole tumor samples out of 73 LOHs positively diagnosed in microdissected samples (34%). With the LOH detection threshold set at -50%, the respective figures were 39 LOHs not diagnosed out of 55 LOHs (71%). Measuring the intensity of the allelic decrease, we showed that the mean decrease of the lost allele is -34% with whole tumor samples and -67% with microdissected samples. The increase in sensitivity of LOH detection with microdissection is associated with the density of stromal cells. This strong improvement in LOH detection in this aggressive type of breast cancer indicates that many other molecular studies performed on heterogeneous solid tumors may benefit from a first step of laser microdissection.