Although anaplastic lymphoma kinase (ALK) has been considered a diagnostic marker specifying a subset of anaplastic large cell lymphomas and inflammatory myofibroblastic tumors (IMTs), the existence of this receptor in some other mesenchymal malignancies has been recently reported. We examined a wider variety of soft tissue tumors to further advance the survey of ALK status in mesenchymal lesions. ALK protein expression was evaluated immunohistochemically with 2 specific antibodies (ALK1 and 5A4) in 249 benign and malignant soft tissue tumors, and the expression of ALK transcripts and 8 types of ALK fusion transcripts was assessed using reverse transcription-polymerase chain reaction (RT-PCR) in 165 and 100 tumors, respectively. Moreover, ALK gene status was analyzed by interphase fluorescence in situ hybridization (FISH) in 17 tumors with ALK expression. Immunohistochemically, ALK protein was detected in 69 cases (28%), including IMTs (4 of 4), rhabdomyosarcomas (4 of 7), various lipogenic tumors (35 of 65), Ewing's sarcoma/peripheral primitive neuroectodermal tumors (6 of 10), malignant fibrous histiocytomas (8 of 37), leiomyosarcomas (3 of 18), and other non-IMT tumors (9 of 108); however, most of these, except the IMTs, displayed merely low-level expression. Although ALK transcripts were identified in 85 (52%) of the 165 cases examined by RT-PCR, the full-length (wild-type) ALK, rather than the truncated or chimeric forms detected in IMTs, predominated in most non-IMT tumors. Except for 2 IMTs, all cases with the expression of ALK messages displayed no detectable ALK fusion transcripts. More than 67% of the cases analyzed by both RT-PCR and immunohistochemical assays demonstrated concordant results. ALK gene amplification was found in 4 non-IMT tumors (2 leiomyosarcomas and 1 case each of rhadomyosarcoma and malignant fibrous histiocytoma) analyzed by FISH, and the rearrangement of this gene was identified in 2 IMTs. The current data expands the variety of non-IMT soft tissue tumors with ALK expression, and warrants further investigation of its underlying molecular mechanisms.